I agree, and the text of the federal marriage amendment is hopelessly unclear on that score. But it's also possible that without a federal amendment, federal judges will likewise usurp "the right of the State to define marriage," by overturning the DMA, and forcing the federal gov't and even states to recognize marriages performed in any state. I'd be in favor of an amendment that made it clear that states were allowed to accept gay marriage, but also allowed to reject it.
It's long, but all the information contained is pertinent to your post.