I've been debating this question with Ed Whelan over on NRO's Bench Memos blog. Setting aside the question whether gay marriage is a good or a bad idea, I argue that FMA proponents have not demonstrated the sort of federal interest that would justify federal intervention, let alone a change to the Constitution. Here are the posts: Ed Whelan, my response, Ed's reply, and my surreply. I'll be on the road today, so if Ed posts a rebuttal, he'll get the last word.

UPDATE: Ed Whelan has the final word here (and I fixed the link to my surreply above).