It’s hard to discuss the social consequences of gay marriage because we don’t have much data to draw from. One way to pose the question in a more “testable” way is to generalize: What are the social consequences of defining sex as a bodily act between two individuals without any expectation or possibility of a third person being produced by the bodily act. For that , we have plenty of data since that has increasingly become the norm since the pill and abortion revolutionized American sex.
In her Adam and Eve After the Pill: Paradoxes of the Sexual Revolution , Mary Eberstadt probes the question of what has happened since the sexual revolution detached nature from nurture. She argues that “the sexual revolution has proved a disaster for many men and women” and “its weight has fallen heaviest on the smallest and weakest shoulders in society - even as it has given extra strength to those already strongest and most predatory” (16). Through the course of the book, Eberstadt examines the consequences of the sexual revolution for women, men, children, young adults.
Among the consequences is a dramatic increase in sexual addictions, which bring all sorts of social evils in their wake: “marriages lost or in tatters; the sexual problems among the addicted; the constant slide, on account of higher tolerance, into ever edgier circles of this hell; the children and teenagers lured into participating in various ways in this awful world in the effort to please romantic partners or exploitive adults.” In short, though the consumption of pornography might be private, “the fallout from some of that consumption is anything but” (60-61).
Gay marriage should be seen as a logical extension of this sexual revolution. And the response to gay marriage advocates might take this form: Since the sexual revolution started by heterosexuals has been so successful, you want to institutionalize a radical form of sexual liberation by making marriage a legal structure that protects sex without the possibility of a third.