John Milbank observes that in recent British debates over gay marriage, “legislators have recognised that it would be intolerable to define gay marriage in terms equivalent to ‘consummation,’ or to permit ‘adultery’ as legitimate ground for gay divorce.” In these decisions, “the legislators have been forced tacitly to admit the different nature of both gay sexuality and of gay sociality. But such an admission destroys the assumption behind the legislation and the coherence of what the legislation proposes to enact.”
Milbank doesn’t think it will stop there. If gay adultery has no legal force, then, on the assumption of equality, heterosexual adultery will also cease to be grounds for divorce. After all, “if the binding and loosing of gay and straight marriage are stipulated in different ways,” then the distinctions that gay marriage is designed to eliminate is reinstated. In Milbank’s view, “secular thought will not so readily let go of the demand for absolutely equal rights based on identical definitions. In that case, we face an altogether more drastic prospect. Not only would ‘marriage’ have been redefined so as to include gay marriage, it would inevitably be redefined even for heterosexual people in homosexual terms. Thus ‘consummation’ and ‘adultery’ would cease to be seen as having any relevance to the binding and loosing of straight unions.”
In short, gay marriage is not an expansion of marriage to include a new set of relationships. It undermines marriage as such, and thus tends to make marriage, not just gay marriage, impossible.