Unlike the debate over abortion, which raises truly complex and irresolvable moral issues, I believe that my children and grandchildren will not be voting in elections haunted by the specter of same-sex marriage. This is both for legal and psychological reasons. It seems likely that even conservative judges will find it impossible to accept as legitimate the arguments put forward by opponents of same-sex marriage for denying equality to all citizens. Similarly, like with race and gender, sexual orientation seems to be an issue where most citizens eventually learn that their previous beliefs were just habits of prejudice and not reflections of any deeper moral truth. Even the pious will come to appreciate that they are not harmed in any real way by homosexuality -- gay sex (and marriage) is not sex done unto them. (Cue the Ted Haggard jokes.) Poll after poll shows that the arc of history is bending towards increased acceptance of homosexuality and thus of same-sex marriage.
Personally, I am impatient for the day when every state in the Union must recognize same-sex marriage rights. But what if the most justifiable policy on liberal-constitutional grounds is not the institution of "marriage" increasingly opening to new constituent relationships, but rather a universal institution of "civil union" which fulfills the social and moral aims behind subsidizing the family but is entirely neutral not only to the gender or even to the numbers of the partners, but also to the emotional content of domestic life and the purposes behind contracting domestic partnerships?
The argument for gay marriage beyond mere civil unions is the argument from equality and recognition. The state treats homosexuals unfairly when it denies them access to the same social goods and benefits open to heterosexual couples and fails to treat them fully as equals when it offers them a different public status, even one with the identical package of objective goods and benefits. In fact, conferring a "civil union" status short of marriage publicly marks the unwillingness to extend to homosexuals recognition of them as equal to heterosexuals and is thus a form of stigmatization. Therefore, the only just policy is to level up all the way to full marriage rights.
Well, another way of handling the problem of recognition and status is to "level down." Why not treat "marriage" the way we treat religion: as something protected and possibly subsidized in moderate ways by the state but which the state is not in the business of defining, regulating, honoring or distributing other than for reasons of individual rights and legitimate public interests? Religion is left to persons and communities to define and defend. Questions of fairness or recognition between religions or conceptions of religiosity arise in this arrangement only rarely and at the margins. (Of course, questions of secular vs. religious authority arise all the time, but rarely is the question the state's obligation to recognize a minority religion as a religion but rather the state's right to trump religion, or disagreement on the status of secular or minority religious values.) Similarly, if all the state offered to anyone was legal status as a civil union, then there could be no complaint of unfairness or disrespect by those offered a status less than marriage.
Furthermore, removing the symbolic good of the word "marriage" from the authority of the state may also appeal to conservatives or the religious, as well as some gays troubled by the historical baggage bound up in the institution of "marriage." At least part of what troubles the religious about gay marriage is precisely the collective, public bestowal of recognition on homosexual relationships as the same thing as heterosexual ones. Leveling down, which is essentially the privatization of the commodity of "marriage" as an emotionally and symbolically invested concept, solves this problem. This symbolic commodity can now be the monopoly of every group that wants it. The state doesn't baptize; why should it "marry"? More accurately, the state doesn't say who can be baptized; why should it say who can be "married"? Conservatives and the religious can keep the institution of "marriage" (better yet, "privatize" it!) and merely have to suffer the knowledge that homosexuals can confer social and economic benefits on one another. And if neither side is satisfied by this (i.e., gays and their allies still want the public bestowal by the state of "marriage" as an act of recognition, and conservatives still want to monopolize decision-making over the question of whom the state extends primary social goods to), then liberals can, with a good conscience, claim that they are both demanding something the state does not necessarily have to give them.