Or maybe he didn't support gay marriage then and doesn't support it now, but is scared of a ravenously malicious committee of self-appointed witch hunters who are out to destroy anyone who dares to disagree with them on their pet cause. Or maybe he just did it because certain organizations or people would give him more money for his presidential library, and he still privately disagrees with it.

I'm not sure why we assume that he's not just doing this for political expediency now if we accept that he may have held the previous position for political expediency as well.