You may think you have a useful issue, and it is of some use, but here's the problem. Making same-sex marriage an important issue will give rise to the thought: If Obama loses, it will be because of the stand he took on same-sex marriage. That will cause some people – people in the middle who might go either way — to think:
I don't want to be a part of sending the message that if you support same-sex marriage, you will lose. I'm going to vote for him, not because he did anything that significant in finally getting around to evolving into the position, but because, now that he has, I don't want him punished for it. I'm afraid of how that will affect other politicians in other elections for who-knows-how-many years into the future. I wouldn't have been a single-issue voter on this, but now that it's been centralized as an issue, I don't want it to be remembered as the reason why Obama lost.
Think back to 2008. Why did people vote for Obama? You might suggest: People got too enthusiastic about the idea of the first black President and that kept them from looking deeply enough into whether Obama had sufficient experience or whether he was too far to the left. But another way of thinking about that is: At some point, people didn't want it to seem that what had happened was that Americans rejected him because he is black. They didn't want to be part of creating the message that if you are black, you will lose. I'm not saying that was rational. I'm talking about the emotional pull people feel, an urge that goes unexamined.
The issues in this election should have to do with economics, foreign policy, and the things that fall squarely within a President's responsibility. Obama has a record here, and he should have to defend it, not distract us with a "social" issue. His actual political policy on same-sex marriage isn't even different from Romney's: Leave it to the states. Leave it to the states is a fine — truly excellent — way to package the issue and set it to the side. I would encourage Republicans to do exactly that:
Marriage has long been a matter that belongs to states. Both Obama and Romney know that and know that it is not what the U.S. Presidency is about. They do not differ on the actual policy. It's good that Obama has expressed respect for federalism here. If only he would see the value of federalism more generally instead of continually enlarging the role of the federal government. Let's look at his record of growing federal government at the expense of state and local government and at the expense of private entrepreneurship....
ADDED: The "truly excellent" package won't necessarily stay closed.