In 1995, in one of the more interesting exchanges over the morality of gay marriage, Stephen Macedo debated Robby George & Gerard Bradley in the pages of the Georgetown Law Journal. Noting the emphasis that natural law arguments against gay marriage place on the fertility of heterosexual couplessexual acts, Macedo raised the following question:

What is the point of sex in an infertile marriage? Not procreation; the partners (let us assume) know that they are infertile. If they have sex, it is for pleasure and to express their love, or friendship, or some other shared good. It will be for precisely the same reasons that committed, loving gay couples have sex. Why are these good reasons for sterile or elderly married couples but not for gay and lesbian couples? If, on the other hand, sex detracts from the real goods shared by homosexual couples, and indeed undermines their friendship, this should also be the case for infertile heterosexual couples. Sterile couples' experience of sexual intimacy should be as "private and incommunicable" as that of gays.

George and Bradley denied that their argument [against gay marriage] swept in elderly or infertile couples. Those couples, they argued, were capable of having sex of the reproductive type, even if they were not capable of reproducing. And it is the noninstrumental (and, they argue, self-evident) good of this sexual activity of the reproductive type within the "two-in-one-flesh communion" of marriage that gives marital sex its value, not some instrumental connection between sex and reproduction (or pleasure or bonding or any other good). The argument is surely a subtle one. (If it is an argument at all -- it seems to rest on a sort of take-it-or-leave-it assertion about the nature of marriage which in turn incorporates the old "holes and poles" argument, as one of my colleagues at Fordham used to call crude, anatomical/functionalist arguments against homosexual sex.) In any event, the focus on the (at times counterfactual) possibility of reproduction (the sex acts must be of the reproductive type, even if not fertile) at least runs the risk of encouraging those who do not grasp the full subtlety of the argument to place homosexual sex, sex with contraception, and infertile sex in the same category. Since most people's minds are not as sharp as Robby George's, it is not surprising that the distinction between potentially reproductive sex and "sexual acts of the reproductive type" has escaped even many of those people on the anti-marriage-equality side of the issue. Consequently, many of them have focused on the infertile nature of gay marriages as the distinguishing quality that invalidates them. Relatedly, in an effort to discredit gay parenting and hold up the value of "traditional marriage" (which now, since it excludes step-parents and the divorced, includes fewer and fewer people), some have begun to cast aspersions on adoptive relationships in general, at least by implication.Over at Charles Pierce's Daily Politics blog, Tom Junod describes and dissects this tendency from the perspective of an adoptive parent in an infertile marriage. Here's a taste, but you should really go read the whole thing:

Since my wife and I adopted our daughter, weve come to know many same-sex couples who are also adoptive parents, and it is exactly as proponents of natural marriage fear: it is their prowess as parents, rather than as pro-creators, that turns out to be persuasive. I have come to believe that they have the right to be married because I know that I have the right to be married, and I know that they are the same as me because I know that I have more in common with gay adoptive parents than I do with straight biological ones. In my wife and in me, the self-evident biological purpose of procreation may be broken, but by God, we earn the right to be called parents because of the effort required to raise our child apart from the sacred biological bond...and so they, our friends engaged in the same effort, the same mighty and holy labor, earn the right to be called married. People wonder why public opinion regarding same-sex marriage has shifted so quickly; although I can only answer from my own experience, I can tell you that in my case my recognition of the right of same-sex couples to marry grew directly from the arguments mustered against it, because ultimately I realized they were also mustered against my wife, against me, and against the one person all the pro-marriage protestors and pamphleteers have pledged themselves to protect:My child.