By that logic, I'm sure no one working at Mozilla voted for Barack Obama in 2008 when he publicly held a position you now consider in the same ethical category as that a "racist" would hold.
This is not so far removed from saying Mary Jones shouldn't be CEO of Mozilla because she donated to a campaign against CO2 emission restrictions: Again by analogy with a settled political issue, "She's denying Global Warming. You'd be OK if she denied the Holocaust?"
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.
So you'd vote for a Barack Obama who professed support for racial segregation as long as the other major party candidate did, too?
I'm not arguing against voting pragmatically, I'm arguing that calling opposition to same sex marriage akin to racism today is a nonsensical comparison. Maybe it will look that way in hindsight, but it's not fair to hold anybody to hypothetical future standards.
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.