So what are gay marriage’s opponents really defending, if not some universal, biologically inevitable institution? It’s a particular vision of marriage, rooted in a particular tradition, that establishes a particular sexual ideal.

This ideal holds up the commitment to lifelong fidelity and support by two sexually different human beings—a commitment that involves the mutual surrender, arguably, of their reproductive self-interest—as a uniquely admirable kind of relationship. It holds up the domestic life that can be created only by such unions, in which children grow up in intimate contact with both of their biological parents, as a uniquely admirable approach to child-rearing. And recognizing the difficulty of achieving these goals, it surrounds wedlock with a distinctive set of rituals, sanctions and taboos.

The point of this ideal is not that other relationships have no value, or that only nuclear families can rear children successfully. Rather, it’s that lifelong heterosexual monogamy at its best can offer something distinctive and remarkable—a microcosm of civilization, and an organic connection between human generations—that makes it worthy of distinctive recognition and support.

Ross Douthat, “The Marriage Ideal”