The argument that gay marriage would subtly undermine the unique legitimacy of straight marriage is based upon a fallacy. For heterosexuals, straight marriage would remain the most significant--and only legal social bond. Gay marriage could only delegitimize straight marriage if it were a real alternative to it, and this is clearly not true. To put it bluntly, there's precious little evidence that straights could be persuaded by any law to have sex with--let alone marry--someone of their own sex. The only possible effect of this sort would be to persuade gay men and women who force themselves into heterosexual marriage (often at appalling cost to themselves and their families) to find a focus for their family instincts in a more personally positive environment. But this is clearly a plus, not a minus: gay marriage could both avoid a lot of tortured families and create the possibility for many happier ones. It is not, in short, a denial of family values. It's an extension of them.
...There's a less elaborate argument for gay marriage: it's good for gays. It provides role models for young gay people who, after the exhilaration of coming out, can easily lapse into short-term relationships and insecurity with no tangible goal in sight. My own guess is that most gays would embrace such a goal with as much (if not more) commitment as straights. Even in our society as it is, many lesbian relationships are virtual textbook cases of monogamous commitment. Legal gay marriage could also help bridge the gulf often found between gays and their parents. It could bring the essence of gay life--a gay couple--into the heart of the traditional straight family in a way the family can most understand and the gay offspring can most easily acknowledge. It could do as much to heal the gay-straight rift as any amount of gay rights legislation.
If these arguments sound socially conservative, that's no accident. It's one of the richest ironies of our society's blind spot toward gays that essentially conservative social goals should have the appearance of being so radical. But gay marriage is not a radical step. It avoids the mess of domestic partnership; it is humane; it is conservative in the best sense of the word. It's also about relationships. Given that gay relationships will always exist, what possible social goal is advanced by framing the law to encourage those relationships to be unfaithful, undeveloped, and insecure?
That last paragraph is pretty much the money quote, for me. Assuming I'm preaching to the choir, dear readers, I trust I don't need to recite the litany of cognitive dissonance inherent in much of the radical right's position. This one, however, really tops the list. The way to preserve marriage as a cornerstone of civilization is to ENCOURAGE it among any and all willing parties. The more participants you have in a system, the stronger it tends to be. Duh.