Canada permits same-sex marriages. Vermont permits civil unions. The New York Times prints "celebrations" of those unions each week, while media profiles of well-adjusted "life partners" are all the rage. If, despite all these mechanisms for commitment, homosexual men continue to be famous for their promiscuity, do you really think giving legal sanction to the additional term "spouse" will lead to fidelity?
I don't know what to think. I don't understand the typical gay person's motivation. Being a magnanimous sort, I'd venture to say that they'd treat marriage with no more (and no less) seriousness than most heterosexuals. Given the infidelity and divorce rates among heterosexual married couples, that's not saying much. Marriage is dying in this country. At the very least, it doesn't mean what it used to mean (an understatement).