While I support gay marriage, largely for the reasons Stephen and his first commenter Seth lays out, I don’t have a major problem with Kurtz’ argument.
It’s one thing for society to say that homosexuals have a right to, well, be homosexual. It’s another for it to place its imprimatur on the relationship and say, “We endorse this and put it on par with heterosexual marriage.” Marriage, especially with the current easy divorce laws, is really nothing more than a social stamp on a union. It also conveys certain advantages in terms of taxation (although not always), property rights, and the like. But we grant those priviledges because we as a society have decided that we want to promote the institution. The main rational basis for which is the protection of children. Given that the vast majority–although a shrinking one–of society believes homosexual relations are repugnant, I don’t think they have to give it a ringing endorsement.
So, why do I support it? While I’m not enthusiastic about the idea, I think the argument for homosexual marriage is ultimately the same as for heterosexual marriage: monogamy is better than promiscuity. Not that monogamy isn’t possible without marriage or guaranteed by it. But it does seem more likely. While homosexuals, presumably, aren’t going to have a spate of out-of-wedlock pregnancies from their promiscuity, they are spreading disease. (Also true of promiscuous heterosexuals, although not at the same rate–and heterosexuals can get married under current law.)
So, on libertarian and practical grounds, I think government should get out of the way and legislate on a rational basis. But the conservative argument here isn’t just outdated nonsense, either.