Well... you see, DOMA isn't unconstituional right now.
Since no state defines marriage as anything other than a contract between a man and a woman, it's perfectly fine for a state to say explicity that a marriage is between a man and a woman.
But, now that the Mass. Supreme Court might suddenly create a possibilty of DOMA being unconstitutional, there needs to be a constitutional amendment to make DOMA constitutional. Bah.
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why would marriage laws be left up to the federal government at all?
Because the Republicans completely own the crap out of the federal government... Hence, anything the Republicans want should be done by the federal government. Of course, when the Democrats own the Federal government, then the Democrats are all in favor of having the federal government stick its hand in wherever the fark it can...