Monogamy is good- and marriage without monogamy isn't marriage, no matter what the law says
Heterosexual "serial monogamy" aside, the average straight man has nine sexual partners in his lifetime. The average gay man has over a hundred. And "committed, caring relationships" among gay men don't necessarily constitute an exception.
This is not a good thing. And it should be taken into account before extending the legal definition of marriage to relationships which have very little in common with marriage as it has hitherto been understood.
It's not just about who is getting "married." It's about what marriage is- and whether it matters.