Considering the lay of the land, same sex marriage is likely to be legal in every state before Obama leaves office. He's had very little to do with that, yet I suspect future historians will the legalization of same sex marriage one of his major achievements.
I'd say Biden and Obama both had quite a bit to do with it.
They were the first vice president and president to publically endorse it after all, and they both did it emphatically and with great passion. I'm sure that tipped the scale for many people's perception of the issue.
