As you surely recall, one of the big arguments against state laws allowing gay marriage, and a key motivation behind the DoMA, was that the Full Faith and Credit Clause would force all states to recognize gay marriage if any did. Many state gay marriage laws were explicitly modified in response to those fears, denying out of state couples the ability to drop in for a wedding illegal in their own state then go home and demand recognition their states laws deny. It seems as open and shut as the Equal Protection Clause argument but then, I am not a lawyer.

I find it interesting that, on both burden of proof and the Equal Protection Clause, conservatives and liberals take opposing views of the Constitution on gay marriage, but switch places on guns. I can respect both loose and strict construction views of the Constitution, but going back and forth as convenient strikes me as very self serving. If the Constitution requires TX to accept gay marriage licenses issued in CT, surely it requires CT accept gun licenses issued in TX; it cannot require either unless it requires both, because both rest on the same constitutional foundation.