The current National Review cover story against marriage equality relies almost exclusively on the idea that the "fundamental reason for marriage" is that "the sexual intercourse of men and women regularly produces children":
Same-sex marriage would introduce a new, less justifiable distinction into the law. This new version of marriage would exclude pairs of people who qualify for it in every way except for their lack of a sexual relationship. Elderly brothers who take care of each other; two friends who share a house and bills and even help raise a child after one loses a spouse: Why shouldn’t their relationships, too, be recognized by the government? The traditional conception of marriage holds that however valuable those relationships may be, the fact that they are not oriented toward procreation makes them non-marital. (Note that this is true even if those relationships involve caring for children: We do not treat a grandmother and widowed daughter raising a child together as married because their relationship is not part of an institution oriented toward procreation.)
The core argument here is that marriage is mating. They (although it reads uncannily like Robbie George to me) even use that word. They seem to believe that marriage must be regarded as essentially biological, if we are to rear children properly. In the abstract, circular, Ratzingerian world, this makes sense. In the real world, it simply cannot apply to modern life.
Leaving the countless existing gay families to one side, adoption, artificial insemination, and surrogates all regularly produce children. And there is no actual evidence that children begotten not by parental mating fare worse than those who are. There is even some research suggesting that lesbians are better parents than heterosexual couples. If your concern is children, why does the process by which a couple obtain a child matter more than the quality of that child's upbringing? Rauch demolishes the article:
The article is a mass of non sequiturs. It assumes that if marriage is “for” somethingregulating procreative sexthen using it for anything else must be “against” marriage, which is like saying that if mouths are “for” eating, we mustn’t use them for talking or breathing. It claims (conjecturally) that marriage would not have arisen if not for the fact that men and women make babies, from which it concludes that society has no stake in childless marriages.
It argues that marriage, and a culture of marriage, are good and important, a point on which thoughtful gay-marriage advocates enthusiastically agree. But, of course, our whole argument is that including gays won’t stop marriage from doing the good things it now does, and will probably strengthen marriage and the marriage culture. Maybe we’re wrong. But the editorial doesn’t even bother to engage. It proceeds as if “gay marriage is bad” follows obviously from “straight marriage is good.”
That's because this is about theology, not politics. Which is increasingly what NRO conservatism is about - and a cramped, defensive, cloistered, out-dated theology at that.