This will be a disaster in places like Maryland. No one expects the congregants of Pastor Burns’ church to run out and vote for Romney, but they’re going to be a lot less enthused about Obama — and a lack of enthusiasm in his base will put an end to hopes of a second term. Maryland is a safe state for Democrats under normal conditions; it last went Republican in 1988, and Obama won it by 25 points in 2008. However, the same reason for that large margin of victory shows the risk for Obama, which is that 30% of the vote in the state comes from the African-American community that Pastor Burns and others serve.
Here's anecdotal support for that. A friend volunteering at Maryland Marriage Alliance headquarters overheard some African American volunteers discussing the president's stand. One woman came in and announced loudly to no one in particular, "Well, President Obama has sold his soul for votes!"
Later I asked this lady if she thought his support of homosexual marriage would sway a lot of people. She said she didn't know what she would do, because she didn't like Romney so she'd probably not vote. Today another lady came in expressing consternation, but said she didn't know what to do because "Romney wants to fire people."