I'd still bet anything that it falls to the wayside if he is re-elected;
You mean you think he won't try to pass a federal marriage law after all?
Oh wait, that doesn't exist in the first place.
So what do you think Obama won't do? He already opposes DOMA, state constitutional bans, etc. This was a personal evolution, and it carries the power of the pulpit with it, but... his admin already was doing what it could at the federal level to deal with state marriage law - which is where marriage law lies.
Just as a refresher - Obama cannot pass a law in the sense of legislating new laws. He can sign bills passed by Congress though.
That particular ball is in Congress's ballpark.
Now he could try something shady like writing an executive order to grant SSM privileges at a federal level and try to skirt DOMA, but that might incur SCOTUS review which might not work out favourably.
What he could have done would have been to really push for marriage equality in the first year of his office, when there was a solid Senate majority and a very strong Congressional Democratic presence, along with some liberal coastal state GOP'ers who might have crossed the aisle.
But Obama was also trying to read the pattern to see how he should tack for the general election - by seeing the way the national GOP went nuts tripping over their own dicks to speak against SSM and other idiotic social conservative issues - he made a calculated gamble to more vocally support marriage equality (the social conservatives would not vote for him anyway, but once he knew that the moderates largely DGAF about SSM...)
I think he's made the right choice, and this should help him more than hinder in the months ahead.