Is polygamy the next same-sex marriage? Fundamentalist Mormons, Muslims, and others argue that federal and state statutes on religious freedom protect the practice. Some liberals have joined the cause, using arguments about sexual liberty, equality, and autonomy. Traditional criminal prohibitions against adultery, fornication, abortion, contraception, and sodomy, they argue, have all now been eclipsed. Criminal prohibitions against polygamy must be repealed, too.

Cases challenging the constitutionality of anti-polygamy laws have been filed in the United States, Canada, and Europe. A federal district court in Utah has declared the state’s laws against polygamy partly unconstitutional. The first rounds of public debate about legalizing polygamy have appeared in newspapers, journals, and blogs. Shows like Big Love and Sister Wives portray polygamous arrangements as happy and normal, affecting our cultural imagination and stoking sympathy for what was only recently thought of by most as a domestic aberration. With polygamy becoming the newest front in the culture wars, we seem to be on the same trajectory that brought us same-sex marriage.

But this is not the case. Recognition of same-sex marriage does not make legalization of polygamy inevitable. Whereas the prohibition on same-sex relationships rose and fell with the legal influence of Christianity, the Western legal tradition’s prohibition of polygamy is based on deep pre-Christian precedents, and its arguments against polygamy find strong expression in post-Christian, Enlightenment thought. This indicates that the secularization of the West need not obscure the strong reasons to reject polygamy. While we may now live in an era of intense sexual liberty, we remain attentive to social justice. This allows our secular contemporaries to recognize the unique threat that polygamy poses to society as a whole, especially to women and children.

The Hebrew Bible counts more than two dozen polygamists among the heroes of the faith. The Mosaic law countenanced polygamy in cases of seduction, enslavement, poverty, famine, or premature death of one’s married brother. The New Testament does not contain an explicit prohibition of polygamy, though it implies one in Jesus’s talk of two becoming one flesh in marriage and in Paul’s instruction that a church leader should be “the husband of one wife.” But it was the pagan Greeks of the fifth and fourth centuries b.c. who first clearly denounced polygamy to be a form of “domestic tyranny.” And it was a pagan Roman emperor who first criminalized polygamy in 258 a.d., more than a century before the establishment of Christianity and nearly a millennium before church authorities issued comparably firm proscriptions of their own.

The high medieval Catholic Church and early modern Protestant churches solidified the strong rejection of polygamy characteristic of the pre-­Christian Greco-Roman world. But Christianity was a carrier, not an inventor, of the West’s criminal ban on polygamy. When Augustine wrote about the morality of polygamy he justified it in the lives of the biblical patriarchs, but not for the people of his day: “A plurality of wives was no crime when it was the custom; and it is a crime now because it is no longer the custom.”

Because Christianity adopted rather than ­initiated the prohibition of polygamy, it’s not surprising that nations in Europe and North America today remain firmly opposed to the arrangement, even as they distance themselves from their Christian past when it comes to other moral matters. Indeed, some of the strongest Western arguments against polygamy came from Enlightenment liberals who rejected the Christian domination of social mores, but also rejected polygamy as a betrayal of reason, nature, utility, fairness, liberty, and common sense. They marshalled their strongest anti-polygamy arguments not against secular sexual libertines, but against Christian splinter groups that were pressing the case for polygamy. In the nineteenth century, it was Christian liberals and missionaries, Mormons, and other religious groups, not progressives, who were demanding its legalization.

Traditional arguments against same-sex relations begin with the observation that gay and lesbian sex is by nature “non-­generative.” However consensual and loving, same-sex intimacy cannot produce a child, which is the ultimate end and good of sexual intercourse. Having a child, classical and Christian authors alike argued, is essential for the preservation of the human race and the perpetuation of one’s own family name, business, identity, memory, and more. Moreover, these writers argued, “even the beasts” do not engage in same-sex activities, despite their lack of reason and conscience. Many animals kill and eat each other, take each other’s homes, food, mates, and offspring—acts that humans see as wrong. But even the beasts, following natural instincts alone, know that same-sex activities are unnatural.

Arguments based on the generative end of the sexual act do not at all apply to polygamy, however. Procreation is enhanced by having multiple wives. A single male having many mates is not only known in nature but is the predominant form of reproduction in most animals, including more than 95 percent of all higher primates. Pairing birds, voles, and a few other animals are the monogamous exception. The human body is not only capable of having multiple sex partners but a man can impregnate several women in a night, though a woman can have only one pregnancy at a time no matter how many men she takes into her bed. That’s why St. Augustine and later Western sages such as Hugo Grotius thought that, even if proscribed, one man with many wives is a “natural” form of procreation.

The arguments against polygamy based on nature have a foundation other than the procreative end of the sexual act. Nearly eight centuries ago, Thomas Aquinas put forward what would become a commonplace of Western thought and law thereafter, especially among Enlightenment liberals and common-law jurists. Human beings, Thomas argued, are distinct among the animals in having perennial sex drives rather than annual mating seasons. They produce vulnerable babies who need the support of both their mother and father for an extended period. Women bond naturally with children; men do so only if they are certain of their paternity. Exclusive and enduring monogamous unions are thus the fitting way that humans can at once have regular sex, paternal certainty, and mutual caretaking for their young children. Humans have learned by natural inclination and hard experience that monogamy best accords with human needs.

Later Catholic and Protestant writers argued that polygamy violates not only the natural law but also the natural rights of wives and children. Calvinist theologian and jurist Theodore Beza stated this argument clearly almost five centuries ago. Taking the Ten Commandments as his guide, he argued that polygamy violates the commandments against adultery, theft, false testimony, and coveting all at once.

Each of these natural duties has a correlative natural right that polygamy breaches. It violates the first wife’s natural rights to marital fidelity and trust, to ongoing marital property and material security, and to contractual expectations and reliance on her husband’s fidelity to the marriage contract. It runs counter to the children’s natural rights to proper support, inheritance, and the undiluted care, nurture, and education of their father and mother together. And polygamy breaches a neighbor’s rights to have an equal opportunity to marry without having most of the eligible women horded in one harem or having his own wife or daughters subject to the covetous privations of a powerful polygamous neighbor. Polygamy was thus doubly unnatural, Beza concluded—a violation of natural law and natural rights alike.

Enlightenment liberals and common-law jurists from the seventeenth century onward drew directly on these traditional arguments, even if they rejected Christianity. Most liberals posited natural rights as “inherent” in human nature or the state of nature rather than commanded in the Bible or the order of creation. But they came to the same conclusion: Polygamy violates the natural rights of women and children.

Seventeenth-century English philosopher John Locke, for example, regarded polygamy as a violation of the natural-born equality of men and women, as well as the natural rights of children to be properly nurtured and fully supported by both their mother and father. For Locke, the natural laws favoring monogamy trumped any religious arguments for polygamy, and he would allow no religious liberty exemptions from criminal bans of it. A century later, leading common-law jurist William Blackstone condemned polygamy as a “singularly barbaric” violation of the reciprocal natural rights and duties of husbands and wives. Polygamy, for him, was a grave offense against public health and public order. Scottish philosophers Henry Home and David Hume argued that polygamy would breed tyrannical patriarchy or servile submissiveness in children. Children of polygamy—whose mothers are deprecated, whose stepmothers are hostile, and whose fathers are distant and distracted—simply cannot learn the healthy balances of authority and liberty, equality and respect, and property and responsibility that they need to survive, let alone thrive, in a democratic society. For Home and Hume, and many American writers who echoed them, polygamy undermines the ­common good.

The Western tradition developed another line of argument against polygamy that turned on its potential to do unjust harms. Some 1,800 years ago, ancient Jewish rabbis and early Church Fathers alike warned that polygamy was “trouble”—the literal meaning of the Hebrew term for a “second wife” (tzarah). They observed that it brought grief to the most noble and God-fearing men and women of the Bible—Abraham with Sarah and Hagar, Jacob with Rachel and Leah, Elkanah with Hannah and Peninnah. These biblical polygamists suffered bitter rivalry between their wives, bitter disputes among their children over inheritance, deadly competition among the half-siblings that ultimately escalated to incest, adultery, kidnapping, enslavement, banishment, and more. Think of the great King David who lustfully murdered Bathsheba’s husband to add her to his already ample harem. Or think of King Solomon with his thousand wives and concubines who led him into idolatry, and whose children ended up raping, abducting, and killing each other, precipitating civil war in ancient Israel.

A millennium later, Bishop William of Auvergne, commenting on Middle Eastern Muslim polygamy, argued that this “bent love” harmed women, because they are reduced to rival slaves within the household, exploited for sex with an increasingly sterile and distracted husband, sometimes deprived of the children they do produce, and forced to fend for themselves and their children when other women and children are added to the household against their wishes. Children are harmed because their chances of birth and survival are diminished by their calculating fathers who might contracept, abort, smother, or sell them, and by their mothers who sometimes lack the resources, support, and protection to bring them to term, let alone to adulthood. Men are harmed because they do not have the time, energy, or resources to support their polygamous households and because their minds and hearts cannot rest if they are always on the lookout for another woman to add to their harems—or for dangerous men abroad who might abduct their women. Finally, societies are harmed because polygamy results in too many unattached men who become menaces to public order and morality. Moreover, the complex extended families create ad hoc seats of domestic power based on numeric superiority rather than legitimate political succession or election.

European critics of polygamy faced a real-life illustration of its dangers in the sixteenth century. In the town of Münster, a group of young men, giddy with lust and theocratic pretensions, combined charisma, brutality, and biblical platitudes to force a gullible Christian community to adopt their utopian vision of biblical polygamy. Old couples were forced to end their marriages and start again. Young women were coerced into premature and unwanted marriages with older men; even prepubescent girls were fair game and were raped to death. Husbands collected wives like spiritual trophies, measuring their faith by the size of their harems and nurseries. Wives were used and then spurned when they were pregnant or nursing or when the next wife was added to the harem. Polygamous households became filled with bickering wives and children, who were then cowed into silence with threats of the sword. Wives who still objected, or who rejected their husband’s sexual advances to protest the unwanted polygamy, were summarily executed. Dissenters and critics were banished or executed.

We can see a similar pattern of dysfunction, albeit less flagrant, in the polygamous communities scattered about the Western world. They feature higher-than-average incidences of arranged, coerced, and underage marriages of young girls to older men; rape and statutory rape; and wife and child abuse. The women and children in polygamous households are often socially and educationally deprived. Young boys and poorer men have to compete for fewer brides. Oversized polygamous families commonly abuse social welfare programs, and polygamous communities are often socially isolated and combine religious and communal authority in coercive ways.

In the non-Western world, most polygamous cultures also feature social dysfunctions. After completing an exhaustive study of polygamy in 170 nations, Brown University political scientist Rose McDermott concludes, regardless of “whether it is practiced in a Western democracy or sub-Saharan Africa, polygamy produces harmful effects that ripple throughout a society.” Polygamous communities suffer from increased levels of physical and sexual abuse against women, increased rates of maternal mortality, shortened female life expectancy, lower levels of education for girls and boys, lower levels of equality for women, higher levels of discrimination against women, increased rates of female genital mutilation, increased rates of trafficking in women, and decreased levels of civil and political liberties for all citizens. The law’s prohibition of polygamy would seem based on a sound judgment about the harm its practice inflicts on the most vulnerable.

Skeptics of this line of reasoning are quick to point out that monogamous households are filled with many ugly harms, too: wife and child abuse, infidelity, abandonment, welfare abuses, and more. Yet these failures have not led to the abolition of monogamy but only to closer regulation to prevent harms and punish those responsible. Why not do the same for polygamous households? If polygamous wives or children really do suffer from increased levels of abuse, neglect, or deprivation, why not build protections into the law and enforce them scrupulously? If religious communities isolate their members, making them more vulnerable to abuse, why not make polygamy more mainstream, transparent, and accountable? If Big Love and Sister Wives can make the polygamous family work, why can’t everyone else be given a fair chance?

But this is to build the law upon the unique resources available to the powerful, not the more typical needs of the vulnerable. We can imagine a legal regime allowing polygamy when three or more well-educated parties—similar in wealth, ability, and opportunity, eyes and doors wide open—choose to enter into a union. They have the wherewithal to calculate and negotiate the costs and benefits, and the advantages and disadvantages. More important still, they can protect themselves through prenuptial and postnuptial contracts and through their own independent means, hiring lawyers, accountants, private investigators, and security guards to help them if their partners betray or endanger them or their children. For these exceptional sorts of people with lots of resources, the state prohibition of polygamy hardly seems necessary.

But the law must answer to the needs of the typical case, not the exceptional one. And throughout Western history and still today, the typical case of polygamy too often involves vulnerable parties who do not have the knowledge, resources, or connections to secure the kind of self-protection and self-help available to a “sister wife.” Every Western nation has general laws on the books against wife and child abuse; coerced marriage and statutory rape of young girls; depriving children of food, shelter, and education; welfare abuse; and more. Yet these laws provide too little support and protection for those made vulnerable by polygamy. It’s not a sound principle of justice to make vulnerable people more vulnerable just to accommodate the desires of the powerful to undertake experiments in domestic living.

Traditional laws against polygamy are more than just prudential prophylactics against harm. They also play an important symbolic role and teaching function. Laws against polygamy have been part of a broader set of family laws designed to support the classical Western ideal of the monogamous family. Aristotle and the Roman Stoics called the union of husband and wife, and parent and child, the “foundation of the polis” and “the private font of public virtue.” According to the Church Fathers and medieval Catholics, the monogamous household is the “seedbed” of the city, “the force that welds society together.” Early modern Protestants and Anglo-­American common lawyers spoke of the stable marital household as a “little church,” a “little commonwealth,” the first school of love and justice, nurture and education, charity and citizenship. John Locke and other Enlightenment philosophers treated marriage as “the first contract,” and the “deepest font” of liberty, equality, and fraternity.

For all of our new cultural emphasis on liberty and autonomy—or wariness about totalitarian power—we still look to the law to promote well-being. As the legal doctrine has it, the state properly concerns itself with the “health, safety, and welfare” of its citizens, which means discouraging and even criminalizing activities that harm individuals and the body politic.

In our time, the law has backed away from many traditional norms for sex, marriage, and family life, reflecting a social consensus that shrinks from moral absolutes and encourages a nonjudgmental attitude toward personal decisions about sex and relationships. Nevertheless, the teaching function of the law remains. We still “nudge” citizens toward certain ways of life. The state does not require its citizens to get married, but it encourages them to do so. It provides state marital licenses, tax and Social Security incentives, spousal evidentiary and health-care privileges, and hundreds of additional federal and state benefits and incentives that turn on marital status. And while the state rarely prosecutes polygamy simply on its own, it puts in place powerful deterrents. There is no funding, facilitation, licenses, or welfare support for polygamy. When combined with other crimes, polygamy is still prosecuted.

In the aftermath of Obergefell, we can be tempted to think polygamy is inevitable. The reasoning the Supreme Court majority gave for finding a constitutional right to same-sex marriage could be deployed to find a right to plural marriage. But a just legal regime cannot be founded on abstract constitutional logic alone. The network of specific laws and judgments is influenced by (and influence) complex moral and cultural systems. Our legal systems in the West historically censured homosexuality and polygamy, but for very different reasons, and the reasons against polygamy remain in place. Perhaps this stems from the fact that the Christian culture in the West had to grapple with the role of polygamy in the Old Testament—and that Christian and para-Christian sects have revived it on occasion. Whatever its cause, our legal tradition is not nearly as unmanned in the face of polygamy as it seems to be when it comes to the sexual revolution more generally.

At this point all Western nations continue to proscribe polygamy, even as they have accepted sexual liberty and, in many places, same-sex marriage. There are good reasons for this. Unlike gay rights, which can be portrayed as an expansion of freedom at no cost to others, the ample dangers posed by polygamy, dangers traditional commentators point out, still seem real. Our collective sense that polygamy is wrong rests on sound intuitions about ways in which polygamy rewards the powerful—men capable of winning the competition for wives—while harming the vulnerable, which includes women, children, and men less competitive in a winner-take-all mating market.

We have every reason to believe this presumption against polygamy will continue, regardless of how liberalized we become in other matters of sex and family structure. A great deal of evidence shows that most men and women alike are instinctively attracted to long-term, single-partner intimacy and instinctively repulsed and angered if forced to share their bed and partner with a third party. Despite our wide cultural acceptance of sexual liberty in the West, sexual infidelity still breaks marriages and intimate relationships more than any other cause. Moreover, over the centuries, successful societies have consistently migrated from polygamy toward monogamy, but never in the other direction. Perhaps I’m wrong, and the modern sexual revolution will yield a polygamist’s Stonewall, and then an Obergefell. But if so, that will mean that we don’t care all that much about protecting the vulnerable.

John Witte Jr. is director of the Center for the Study of Law and Religion at Emory University, and author of The Western Case for Monogamy over Polygamy.