What people don't seem to get is that convincing people to vote in elections is about 35% swinging independents, and 65% getting the people who will only vote Dem/Rep to actually show up. The amount of people who are actually up in the air about whether to choose Obama or Romney won't care about this, because anyone who really cares about gay marriage as an issue, enough to make it a voting point, has already picked a side. The only possible way this would hurt Obama is if there are people out there who this convinces to vote in '12 that didn't vote in '08, and that number significantly outnumbers the people who this convinces to vote for Obama in '12 that didn't vote in '08.
It's simple algebra.