There is a general form of reasoning to which I shall give the name argumentum ad consummationem, which runs as follows. Major premise: Sexual attraction and love are determinants of human happiness and should be consummated where sincerely felt. Minor premise: You cannot choose to whom you are sexually attracted, and you cannot choose with whom you fall in love. Conclusion: Whether or not they are chosen, attraction and love should be consummated where sincerely felt. This simplistic syllogism (uncritical in its use of choice, love, sentiment, and sincerity) provides the rational foundation for a culture of often unrestrained, promiscuous, and unfaithful”yet indulgently sentimental”coupling. And it undergirds the push for same-sex marriage on both sides of the Atlantic.

While the United States faces a heated political contest over the definition of marriage, in England and Scotland all of the main political parties agree on the desirability of introducing same-sex marriage. The Conservative party, far from offering resistance, has become a principal advocate. Addressing his annual party conference, Prime Minister David Cameron said, “I don’t support gay marriage despite being a Conservative. I support gay marriage because I’m a Conservative.” Opposition to same-sex marriage now typically is characterized as “homophobic.” This is lazy, and it commits the genetic fallacy by shifting from the rational grounds to the purported non-rational causes and motives for opposition.

Yet critics of homosexual unions overlook the extent to which our societies are addicted to sexualization and sentimentality and are inclined to excuse these factors in the case of heterosexuals. To call gays, lesbians, the transgendered, hermaphrodites, and whoever else fails to be straightforwardly heterosexual to a standard of chastity that has long since been generally abandoned is ridiculous and callous.

Yet those who condemn gays for their sexual immorality also often overlook or excuse heterosexual promiscuity, especially when validated by serial “marriage” among the rich and the conservative. The very foundation of popular sexual mores invites extension to the homosexual case, and that application is now being made aplenty.

We debate the meaning of marriage in the distinctively modern way that has come to prominence with the rise of liberal individualism: namely, with the language of equality and rights. Opposition to such thinking comes from traditional conservatives”who favor social duties over social claims”and from traditional socialists”who see links between the culture of rights and that of consumerism (with their shared, strident insistence on entitlement, recognition, and self-fulfillment).

There is a related way of thinking about society, one that shaped the founding cultures of Greece and Rome: namely, the morality and politics of the common good ( bonum commune ). In this perspective, institutions such as education, law, and marriage are grounded in human nature and focused on shared life. They are rooted in what joins humans in natural communities, not what separates them into sectional interest groups.

The case for education, for example, is not that children have an entitlement to schooling. Rather, education is a necessity for society and a benefit to be shared within it. Similarly, marriage exists for the sake of making and maintaining family life, the roots of which lie in natural complementarities: in male and female of the species joining together one-to-one, with the intention of creating another.

That other, borne of the fusion of his parents’ diverse identities, thereby extends a union of two to a community of several. Marriage recognizes, celebrates, and protects this basic source of human society. It is not a commodity to be bought or an entitlement to be claimed, and its meaning and value were understood long before the idea of rights was ever conceived of, and the escalatory contest over them ever begun.

Disconnecting marriage from the union of one male and one female leaves no principled reason to restrict it to couples. If same-sex, what of multi-partner marriage in hetro-, homo-, or bisexual combinations? And what of the emerging claims of incestuous siblings for marriage rights?

Same-sex advocates are inclined to treat the latter questions as rhetorical absurdities, but the evidence for posing them is not hard to find”and, Not surprisingly, it generally conforms to the argumentum ad consummationem pattern. The Canadian Polyamory Advocacy Association says that it “promotes legal, social, government, and institutional acceptance and support of polyamory, and advances the interests of the Canadian polyamorous community generally,” adding that “We’re here because we have a right to live with the people we love.”

There is also creeping advocacy of sibling marriage, particularly in northern European countries and in North America where it is an increasing topic of discussion within the GSA (genetic sexual attraction) movement. In 2002, the homosexual, wildly self-promotional, and self-confessedly depressive opinion columnist Johann Hari (later exposed as a plagiarist) published an article in the Guardian headed “Forbidden Love” and subtitled “Can sex between close relatives ever be acceptable?”

Hari’s answer is at best uncertain, and the article ends with what appears to be a rhetorical question: “In any case, we must acknowledge that, with the rise of contraception, we have succeeded in separating sex from reproduction. Another unashamed participant in incest discovered in a chatroom, calling herself ‘daddysgirl,’ insisted: ‘We would never have a baby, it would be all screwed up and wrong. I use the coil.’ So has a window opened for ‘safe’ incest? And if so, is our visceral disgust just a remnant from a vanishing age?”

So far, so narrowly heterosexual, perhaps because Hari is cognizant of the fact that his liberal readership might not yet be ready for homosexuality plus incest. In the investigation of Hari’s journalistic felonies, it emerged that he had been operating on the internet under the pseudonym David Rose. From that address in 2006 a narrative was sent with the subject heading “How My Little Brother Learned to Be a Whore,” in the body of which was a story about how one black teenager sodomizes and pimps his younger brother. Hari was forced to acknowledge the pseudonym and has never countered the allegation of authoring gay incest porn. All this while writing under his own name in a national newspaper inviting readers to consider whether incest might be morally acceptable.

We must acknowledge that if desire and love can take a man to his brother, or a woman to her sister, et cetera, then argumentum ad consummationem sets the foundation for incestuous marriage, as it does for polyamorous unions”just as it already has for same-sex unions. Indeed, contributors to polyamory and GSA forums frequently cite gay advances as precedent and inspiration.

The best case against same-sex marriage is a positive argument: for marriage as the cultural formulation of a natural union, one that needs to be protected from the twin distortions of shallow sentimentalism and animal lust. This is an argument from natural law and so connects to the significant documents of both American and Scottish law.

As Viscount Stair, Scotland’s greatest jurist, wrote in the Institutions of the Laws of Scotland (1681): “Obligations arising from voluntary engagement take their rule and substance from the will of man and may be framed and composed at his pleasure . . . but so cannot marriage, wherein it is not in the power of the parties, though of common consent to alter any substantial . . . marriage arises from the law of nature and it is given as the very example of the Natural law.” Today, however, we are seeing that in the marketplace of sentimental and erotic entitlement, choice is sovereign and is no respecter of nature or custom.

John Haldane is professor of philosophy at the University of Saint Andrews.