Did you read the papers? The danger is not imposition of gay marriage by a state legislature locally nearly as much as the imposition of gay marriage nationally by the judiciary, pretending that an already existing law already imposed gay marriage on us. This doesn't exactly empower the states either.
The only possible response to this federal judicial activism, is to impose federal legal restraint. Trying to hold some theoretical high ground while gay marriage is imposed on all 50 states regardless of their opinion on the matter is not a good example of federalism.
This Amendment goes beyond blocking the Supreme Court from finding a right to gay marriage.
If the goal is only to block the Judiciary, why not just say:
"A State shall have the sole power to decide the legal status of marriages between two adult persons of the same sex."