President Obama completed what he described as the evolution of his position on same-sex marriage. He has long supported civil rights for homosexuals. In the 2008 election he supported civil unions as the state equivalent of marriage, reserving the term "marriage" for religious unions.
This has been my position, too.
However, I also see that it is hard for most people to hold on to a living sense of the difference between the law of the state and the order of the church. Ironically, it is Southern Baptists who seem to have the hardest time with this distinction - even though when they were a sect being persecuted by the state church, Baptists pioneered the church/state wall. In the recent North Carolina vote outlawing all non-marital sexual unions, the victors said that the Bible recognizes only a marriage of a man and a woman, and therefore the state of North Carolina should do the same. They didn't even know they were overthrowing the church/state wall that they themselves had promoted to protect the church.
I think President Obama had a genuine change of heart sometime in the past few months, as he said. But I also think the timing of President Obama's announcement of his support for same-sex marriage was affected by the North Carolina vote. It has been clear for some time that the constitutional amendment was going to pass. This outcome would not have changed if he had announced his support of same-sex marriage before the vote. But I also think the recent series of the statements supporting same-sex marriage by Vice President Biden and other cabinet officers was part of a planned roll-out of the president's new position.
I think the success of legal gay marriage all over the United States is now inevitable. I think the pace will be quick in half the country, slow in the other half - that is, in my half. But the demographic writing is on the wall - the younger generation not only supports same-sex marriage, they don't understand why the old people are against it. And this is true of young Republicans, even young religious Republicans, just as it is of most young people.