Conservative Charles Krauthammer has a good Washington Post piece on Obama's marriage equality arguments. Krauthammer says that there are two main arguments for it, one based on empathy and one based on rights. The President initially adopted the former, but has moved to the language of the latter. The latter position then makes it untenable to allow states to decide for themselves (which remains the president's position) and paints opponents as bigots (a politically difficult position).
Krauthammer is wrong on his facts in the piece in his claim (and oft-repeated by the other side, though erroneous) that marriage has always been heterosexual in every society and through all time. There have been culturally, legally, and religiously accepted same-sex relationships as much historical and anthropological research demonstrates.