Currently, marriage consists of a default package of contractual rights given from one partner to another (e.g. default will, medical proxy) and I have no problem extending this to gays. In fact, this part of marriage can already be pieced together with a few legal documents.
Marriage also consists of laws which force single people to subsidize married couples, for instance the non-working spouse of a worker gets SS benefits that neither spouse paid for. Gay marriage supporters are in the unique position of thinking about this issue and recognizing the unfairness of it. But rather than advocating for fairness, they simply want to be allowed to join the favored class. I'll draw an analogy to the civil rights: think of an Asian American in 1960 saying "I'm not against Jim Crow, I just want Asians to be classified as White rather than Colored."
As a single person, I'm in the same boat as gay couples. Gay marriage supporters want to throw me overboard in order to join the favored group, forcing me to subsidize a slightly larger group of people. Why should I do anything but oppose that?
Marriage fairness (i.e., no subsidies), I'll get behind. Gay marriage, no.
[edit: I thought my last line was clear enough, but from the responses it apparently was not. I favor ending marriage, or at least ending the subsidies for it. I also favor ending the penalties for it, which apply in certain circumstances.]