I think some gay people want to marry because a) they think they have the right and b) they think they have found the right partner. I oppose same-sex marriage but I don’t assume bad faith on the part of all those advocating in its behalf. So far, arguments for same-sex marriage tend to restrict it to two people, a couple, and when asked why, they tend to fumble a bit and say, well, that’s what marriage is, two people. They see the two people part as essential but not the opposite sex part.
But of course, if marriage is redefined as a commitment between two people, it will be hard to argue that it shouldn’t go for three people, perhaps two of one sex and one of the other.
I don’t mention this as a scare tactic. (“If we allow THIS, next thing you know, they’ll want THAT.”) I would oppose same-sex marriage even if I were convinced that marriage would be redefined no further. It’s bad enough in itself. But once we take children out of marriage—they’re just a feature some like but others don’t—it is not obvious why marriage should be limited to couples.