Its the full faith and credit clause. The Left's strategy is to obtain favorable rulings in a few states and then go back to the other states and then challenge the constitutionality of the ban under DOMA. They can't challenge it until they have standing to do so. Which is what they are trying to set up, a federal test case. Its just a matter of time til that happens.
Again, back to my original premise, which you are avoiding like the plague. How is it conservative to amend the constitution to address something that is a mere speculative possibility which is presently covered by a Federal statute?