Sometimes you have to vote for the lesser evil. It's pragmatism. Choose between the person who will try and stop gay marriage from happening by enshrining it in law or the person who won't actively stop it (or someone who has no chance of getting in). Seems quite obvious.

To expand on that, I think a lot of people confuse the Free Software movement with a movement for "ideological purity". No, it isn't about having the most social justice points, it's about creating and sharing free software, anything else is irrelevant. Even if you're a convicted murderer (ReiserFS), although you can't really commit code from prison.

Of course we want to encourage as many people as possible to contribute and therefore anything that pushes large groups of people away (for no good reason) is bad.