In any case, progressives could very well prevail on gay marriage at the federal level eventually, although not without a fight. And taking up arms anew are Sherif Girgis, Robert George, and Ryan Anderson, in their new paper, "What is Marriage":
This is basically the argument I've made against same-sex marriage following the passage of Prop. 8 in November 2008.
Real Marriage Is—And Is Only—The Union of Husband and Wife
As many people acknowledge, marriage involves: first, a comprehensive union of spouses; second, a special link to children; and third, norms of permanence, monogamy, and exclusivity. All three elements point to the conjugal understanding
1. Comprehensive Union
Marriage is distinguished from every other form of friendship inasmuch as it is comprehensive. It involves a sharing of lives and resources, and a union of minds and wills—hence, among other things, the requirement of consent for forming a marriage. But on the conjugal view, it also includes organic bodily union. This is because the body is a real part of the person, not just his costume, vehicle, or property. Human beings are not properly understood as nonbodily persons—minds, ghosts, consciousnesses—that inhabit and use nonpersonal bodies. After all, if someone ruins your car, he vandalizes your property, but if he amputates your leg, he injures you. Because the body is an inherent part of the human person, there is a difference in kind between vandalism and violation; between destruction of property and mutilation of bodies.
Likewise, because our bodies are truly aspects of us as persons, any union of two people that did not involve organic bodily union would not be comprehensive—it would leave out an important part of each person’s being. Because persons are body‐mind composites, a bodily union extends the relationship of two friends along an entirely new dimension of their being as persons. If two people want to unite in the comprehensive way proper to marriage, they must (among other things) unite organically—that is, in the bodily dimension of their being.
This necessity of bodily union can be seen most clearly by imagining the alternatives. Suppose that Michael and Michelle build their relationship not on sexual exclusivity, but on tennis exclusivity. They pledge to play tennis with each other, and only with each other, until death do them part. Are they thereby married? No. Substitute for tennis any nonsexual activity at all, and they still aren’t married: Sexual exclusivity — exclusivity with respect to a specific kind of bodily union—is required. But what is it about sexual intercourse that makes it uniquely capable of creating bodily union? People’s bodies can touch and interact in all sorts of ways, so why does only sexual union make bodies in any significant sense “one flesh”? Our organs—our heart and stomach, for example—are parts of one body because they are coordinated, along with other parts, for a common biological purpose of the whole: our biological life. It follows that for two individuals to unite organically, and thus bodily, their bodies must be coordinated for some biological purpose of the whole.
Here is another way of looking at it. Union on any plane — bodily, mental, or whatever—involves mutual coordination on that plane, toward a good on that plane. When Einstein and Bohr discussed a physics problem, they coordinated intellectually for an intellectual good, truth. And the intellectual union they enjoyed was real, whether or not its ultimate target (in this case, a theoretical solution) was reached—assuming, as we safely can, that both Einstein and Bohr were honestly seeking truth and not merely pretending while engaging in deception or other acts which would make their apparent intellectual union only an illusion.
By extension, bodily union involves mutual coordination toward a bodily good—which is realized only through coitus. And this union occurs even when conception, the bodily good toward which sexual intercourse as a biological function is oriented, does not occur. In other words, organic bodily unity is achieved when a man and woman coordinate to perform an act of the kind that causes conception. This act is traditionally called the act of generation or the generative act; if (and only if) it is a free and loving expression of the spouses’ permanent and exclusive commitment, then it is also a marital act.
Because interpersonal unions are valuable in themselves, and not merely as means to other ends, a husband and wife’s loving bodily union in coitus and the special kind of relationship to which it is integral are valuable whether or not conception results and even when conception is not sought. But two men or two women cannot achieve organic bodily union since there is no bodily good or function toward which their bodies can coordinate, reproduction being the only candidate. This is a clear sense in which their union cannot be marital, if marital means comprehensive and comprehensive means, among other things, bodily.
2. Special Link to Children
Most people accept that marriage is also deeply—indeed, in an important sense, uniquely—oriented to having and rearing children. That is, it is the kind of relationship that by its nature is oriented to, and enriched by, the bearing and rearing of children. But how can this be true, and what does it tell us about the structure of marriage?
It is clear that merely committing to rear children together, or even actually doing so, is not enough to make a relationship a marriage — to make it the kind of relationship that is by its nature oriented to bearing and rearing children. If three monks agreed to care for an orphan, or if two elderly brothers began caring for their late sister’s son, they would not thereby become spouses. It is also clear that having children is not necessary to being married; newlyweds do not become spouses only when their first child comes along. Anglo‐American legal tradition has for centuries regarded coitus, and not the conception or birth of a child, as the event that consummates a marriage. Furthermore, this tradition has never denied that childless marriages were true marriages ...
There's a rebuttal from Kenji Yoshino at Slate, "The Best Argument Against Gay Marriage: And Why it Fails." And then the "What is Marriage" authors respond: "The Argument Against Gay Marriage: And Why it Doesn’t Fail."