Obama's reluctance to embrace gay marriage, putting him among a minority of Americans in national polls, appears part of a broader effort to avoid alienating voters in battleground states, like Ohio and Nevada, where majorities have traditionally shown less support for the unions than voters overall.
It is the peculiar arrogance of leaders in a two-party political system that they can break promises (e.g. "I'm a fierce advocate for gay and lesbian Americans") and expect that the group they hoodwinked will take no notice, because the only alternative is beyond the pale.
If Obama keeps up with his Strom Thurmond shtick, he runs a real risk of losing New York and Vermont, whose people decidedly support same-sex marriage, to a liberal third-party candidate. That would be 32 electoral votes lost, compared to the 24 in Ohio and Nevada.