That's their Achilles Heel.
"Despite the basic rule that a marriage valid where contracted is valid everywhere, the courts and validation statutes have universally recognized a number of exceptions, which may be condensed and simply stated as follows: A marriage valid where contracted will nevertheless not be recognized as valid in the forum state if such recognition would be contrary to a strong public policy of the forum state."
BINGO!
Thirty-seven States with DMA's in place, eight States working on Amending their State's constitution to define marriage as one woman, one man.
If the people of California, Massachusetts, Vermont, or Hawaii wish to allow same sex marriages, that's their constitutional right, but they can't shove it down the collective throats of the remaining States.