Obama On Marriage

Reihan wonders about Obama's stance on gay rights:

So as of now, Obama has staked out a strongly Sullivanian position, yet my understanding is that he personally opposes same-sex marriage. I’d be eager to know why does this parallel his trade rhetoric, where things get amplified and overheated in the course of a campaign?

I have yet to figure out why exactly, except as either an amorphous respect for tradition or as a political necessity. Or both.