I understand the symbolism of President Obama's statement in favor of gay marriage, but his current position is illogical. He's personally for gay marriage, but (somehow) is against making it federal law. He says it should be up to the states. So nothing changes.
His position is completely politically motivated. A WSJ/NBC poll has it at 49-40% in support for gay marriage in America, so he comes out for it, but Obama doesn't want to lose states that are against it in November, so he comes up with this irrational position.
His stance also helps with the youth vote. Right now, 18-35 years are not nearly as excited about President Obama as they were heading into the 2008 election. That age group overwhelmingly supports gay marriage.
I'm glad that society has reached the point where it's politically expedient to come out for gay marriage, but it was not a courage statement. If anything, Vice President Biden's comments on Meet the Press put pressure on President Obama to come out in support of gay marriage. If one of them can be called courageous on this issue, it's Joe Biden.
While his position change, coupled with his policy remaining the same, makes for smart politics, it makes for bad governance. His policy lends itself to the potentiality of having 50 different definitions of marriage. It also allows some states to discriminate and withhold the rights of citizens based on sexual preference. That is a dysfunctional policy.