Perhaps the most important conservative/libertarian thinker of the last two decades has changed his mind on gay marriage. Why? Well listen to his interview with Charlie Rose. The relevant exchange takes place at the 17 minute mark. Money quote:

I've changed my mind on this one … the fact is that my wife and I know many same-sex couples and they have great marriages.

That's an important shift in the conservative intelligentsia. And it's, in his description, about commitment and responsibility and not treating divorce as an easy option.