Why would a gay person not support marriage equality?

Inspired by this news story: http://news.yahoo.com/mother-gay-son-asks-santorum-wife-handle-her-053704939.html

I’m always very surprised when I hear of a self-identified gay person who does not support marriage equality. I try to remember that it is unwise to basically essentialize all gay people, but for the life of me, I can’t come up with a good reason why a gay person wouldn’t support it. I know some people are against marriage on principle (maybe they don’t believe in monogamy or have a problem with the historical unequal gender paradigms in marriage as an institution, etc.), but they usually do not wish to ban it altogether – it’s more of a personal view that they do not project onto others. So why would a gay person be any different? It doesn’t help that I’ve never met a gay person who doesn’t support gay marriage, so I have never been able to ask right at the source, as it were. Any thoughts?