Paul Waldman bets that Obama will be the "last Democratic presidential nominee not to support marriage equality":

While Democrats don't have nearly as many litmus tests as Republicans do, this is almost certain to be something that is expected of anyone who wants to be the party's nominee in four years, and forever after. The really interesting question, though, is how long it takes to get a Republican presidential nominee who supports marriage equality. I'm going to guess it'll be 2024.