The main “philosophical” argument against same-sex marriage/marriage equality seems to be that it denies that “sexual complementarity” is at the core of what marriage is.
Some versions of this argument take what I think is an implausible view of the metaphysical status of what they call the “conjugal union” of a man and woman, but that aside, I’m prepared to agree that there is a strong connection between marriage, sexual difference, and procreation. That is, a large part of the reason why marriage exists in the first place is because when men and women have sex, they sometimes produce babies. And society has an interest in ensuring that children enter the world and are raised in a relatively stable context. So in that sense, SSM opponents are right that there is a strong link between sexual difference, procreation, and the purpose of marriage.
So, let’s concede that the coupling of a man and a woman, with offspring to follow, is the “typical” instance of a marriage. I’m even willing to call it the “central” instance if you like. What doesn’t follow from this fact is that marriage can’t be extended “by analogy,” so to speak, to cover other instances that resemble but also differ from this typical one. And of course this is already the case. Couples who can’t or don’t choose to have kids are just as married as couples who do–and no one blinks at this. This doesn’t mean that “man-woman-kid(s)” isn’t still the characteristic form marriage tends to take; it just recognizes that not everyone’s circumstances are the same and that marriage performs functions that are important even without the presence of children. (My wife and I were just as married during the decade prior to having children as we are now.)
Similarly, then, with SSM. It represents another analogical extension of marriage to cover people who depart in some respects from the typical form. People who want the monogamy, fidelity, and permanence of marriage, but whose chosen partner is a member of the same sex, can comfortably fit under the broad umbrella of marriage. This doesn’t require us to deny that there is an important link between marriage and procreation, but rather to recognize that marriage serves multiple ends–not all of which will necessarily be realized in every relationship.
Anti-SSM activists, such as those profiled in today’s New York Times, argue that if we stop enforcing the “norm” of sexual complementarity, we will also undermine the norms of fidelity, monogamy, and permanence. Adam Serwer pointed out that this is an odd argument since these are the values that movement for marriage equality is trying to uphold. But it also strikes me as a kind of category mistake. Heterosexuals aren’t going to stop being attracted to members of the opposite sex because gay people can get married. This isn’t a “norm” that needs to be enforced.* It’s a reality that needs to be recognized and accommodated. But then so is homosexuality. The question is whether marriage is big enough to include both straight and gay couples who want to make lives together–lives that will in many, if not most, cases include the raising of children. I think it is.
*I think it’s worth noting that there’s a case to be made that same-sex marriage could actually strengthen “opposite-sex” marriage, though not necessarily in ways that would make traditionalists happy. Consider this from theologian/biblical scholar L. William Countryman: “Spouses in heterosexual marriages will have much to learn . . . from partners in stable, long-term homosexual relationships. They have long experienced the difficulties of maintaining enduring relationships in a society which is even less supportive of them than of heterosexual couples; and they have had to do it without socially prescribed divisions of roles and labor. . . . On a deeper level, the re-understanding of womanliness and manliness in our changed circumstances will make headway only if the conversation leading toward it includes both heterosexuals and homosexuals” (Countryman, Dirt, Greed, and Sex, p. 260).