I really hate polls and such that try to gauge approval for same-sex marriages. If surveyed, I'd have a hard time answering and it would depend greatly on the specific language of the question.
To be perfectly frank, as a libertarian I'm all for letting you marry the opposite gender, the same gender, no gender, your aunt, or your dog. I might think a few of those a bit suspect, even possibly revolting, but I don't think the law needs to get involved. Banning or endorsing — whichever. The fact that joint tax returns exist make my skin crawl.
I don't know which I'd pick given such a poll, though, because of an important (to me at least) technicality.
I already said I think the distinction is unimportant, for if given my way the state wouldn't be recognizing marriage in any form what-so-ever. (Incidentally, isn't it wonderful how many problems go away when you remove the need to define things specifically?) But if you're going to have a system that recognizes, defines, and encourages, through incentives like said joint tax returns, formal marriage between individuals, well... go for it. Why should heterosexual be the only valid one? I'm all for letting any joint contract being recognized. If that means civil unions, so be it, so long as it is not granted or denied any difference in privileges (this is the part that is rarely going to be honored).
But I'm hesitant to give a favorable reply because, really, I don't want more definitions and complexity. It's bad enough that the law has to define marriage at all, and now we're going to throw same-sex marriage or civil unions into the legal morass? Great.
I know that going ahead and approving such a change, however further complicating things, is a change for the better in the end... but I grit my teeth anytime I find myself going ahead with something simply because the end justifies the means. But I'll save rants on consequentialism for another time.
In short: A thumb up to same-sex marriage/civil unions. Two thumbs up if we can get rid of state-recognized marriage altogether.