I think it was a desperation move that will backfire on Hussein. But the calculation involves more than just numbers of individuals in each category.
Gays are wealthier, more productive, better connected and more activist than Christian Conservative Democrats. They are very much single-issue voters and they were working up a big ol' gay hissy fit over the NC vote yesterday. Christian Conservative Democrats include many blacks who might stay on the plantation for the black President in spite of the gay.
And gay marriage is much more acceptable among the younger demigraphic, something Obama needs to win. It is also a distraction from his economic failures and high unemployment. Obama would like to make this election about social issues, not his record.