It sounds to me like he's using his popularity to press an issue further into the limelight. It's an age old method, especially used by those in the entertainment industry (within which I would also include politics).
Since laws are affected by politicians, and politicians are swayed by polls and perceptions (and marginally by the views of their constituents), it's one more blow for the anti-gay-marriage lobby.
Meanwhile he gets to have his cookie and eat it, too, as there are no rules against co-habiting just because he doesn't have a marriage license. Jolie can't be that concerned, or she'd have walked.
Sorry for stating the blatantly obvious.