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.