First off...he set it up nicely by saying that he THINKS it should be legal, BUT it should be a STATES rights issue.
In other words he played to his gay and lesbian constituency and at the same time championed STATES rights which conservatives can't really argue against.
Now the ball is in Romney's court. The LAST thing Romney wants to get caught up in is stupid social issues that the President doesn't really have any power over anyway. No, Romney wants to talk about the economy...which is the LAST thing Obama wants to discuss.
But guess what? Obama just did to Romney the same thing that Santorum did to Romney. He is forcing Romney's hand to have to take a position against gay marriage.
Look at what Santorum did to Romney in the primaries. He forced Romney far, far right...even though it's pretty clear that Romney isn't a far right guy at all.
Now Obama is doing it too.
I have to say that once you really look at it...it's a brilliant campaign strategy by Obama. He doesn't lose any voters he already had, the conservatives can't argue about States rights, and now Romney is forced to come out against it.