Obama's announcement hasn't moved the polls much:

[F]or all the hubbub, and all the column inches devoted to gaming out the political fallout, Obama's announcement led to little evident movement in the polls, and what movement there has been has been in his favor. That doesn't mean gay marriage can't matter later in the campaign — if it increases turnout among Ohio evangelicals, or leads to much-improved superPAC fundraising for Obama, it could have an impact on the election without having a major effect in national polling — but for now, it doesn't appear to be changing many votes.