The problem comes when gay activists use the full faith and credit clause to export gay marriage from one state to another. That negates the power of a state to ban gay marriage on its own.
I believe they will try this angle and many will fall for it. But the fact is even with that, currently states don’t have to recognize a lot of legal things in other states. One being concealed carry licenses. One state allows it, and it’s legal, the other doesn’t recognize it at all. This is a 2nd Amendment issue and full faith and credit doesn’t apply here. So there is precedent that that argument may not work in every court. If they pick a liberal court however it will.