Obviously, I can't get every single gay person in the country to come here and post first hand which candidate they support, but I think it's pretty darn obvious that if your biggest issue is gay marriage, you're going to most support the candidate who's platform most pushes that agenda in the direction you want.
Here's a tip. Anytime you start to type "obviously", realize that everything that follows it is essentially guaranteed to be baseless and easily refuted.
If it'll make you happy though, sure, assuming your imaginary straw stuffed voter who cares only about Gay Marriage and no other issues, and beyond that cares only about the repeal of DOMA as the single issue in regards to gay marriage, yes, that imaginary person who doesn't exist would vote for Obama.
Shaft probably would too, also Chef from South Park.
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Disclaimer:
To make a long story short, I don't take any responsibility for anything I post here. It's not news, it's not truth, it's not serious. It's parody. It's satire. It's bitter. It's angsty. Your mother's a *****. You like to jack off dogs. That's right, you heard me. You like to grab that dog by the bone and rub it like a ski pole. Your dad? Gay. Your priest? Straight. **** off and let me post. It's not true, it's all in good fun. Now go away.