Wow, I leave for three days and I sure miss a lot...
I believe (though not unshakably) in two principles:
1) freedom of religion
2) separation of church and state
In accordance with "freedom of religion", I believe that gay marraige, polygamy, and various other deviant marital arrangements as purely religious constructs should be legal. I do not condone marital arrangements in which one or more parties are incapable of consent, e.g. marraige with animals or children, or marital arrangements which are likely to lead to material harm to innocents, e.g. incest, but I don't see how gay marraige between consenting adults causes any material harm.
In accordance with "separation of church and state", I believe that laws governing legally recognized marraiges should not be affected by religious beliefs. This raises the question of why we need legally recognized marraiges at all. The supreme court of my state (Washington) has ruled that marraige is a legal institution created to encourage procreation, and therefor marraige can be restricted, for reasons not related to religion, to marraige between a man and a woman. Personally I think the world is grossly overpopulated, and I don't think we should be encouraging procreation at all. I'm all for encouraging child raising, e.g. through adoption, but I don't want to encourage child creation. I have yet to see credible evidence indicating that gay couples are less effective at raising children than are straight couples. So I am in favor of gay marraige.