Here's one of the highlights of the most recent GOP debate, where Mitt Romney made a surprisingly strong case for the Federal Marriage Amendment. I don't think I've ever heard it explained so clearly in so few words:
I believe that the issue of marriage should be decided at the federal level. You might wonder, why is that? Why wouldn't you just let each state make their own decision? And the reason is because people move from state to state, of course, in a society like ours, they have children, as they go to different states, if one state recognizes a marriage and another does not, what's the right of that child? What kind of divorce proceeding potential would there be in a state that didn't recognize the marriage in the first place? There are - marriage is a status, it's not an activity that goes on within the walls of a state, and as a result, marriage status relationships should be constant across the country.