Here's the point. Of the whites who oppose gay marriage, the large majority are already voting Republican. That is not necessarily true of Latinos and certainly not true with Blacks. If Latinos are voting 2:1 or 3:1 for Obama while being split on gay marriage, obviously there is a lot of pro-Obama and anti-gay marriage overlap. The question is whether this will affect the vote of many of them, but imho it won't.
It's very unlikely there will be any shifts in the African-American community. Turnout might be down some, but I highly doubt the % of the AA vote for Obama is going to drop more than 1-2 points. Obama - pre-evolution - probably was realistically sitting at 75% with Latinos. Even if it drops to 65%, he'll be alright. He's got plenty of leeway and he knows it.