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."
If this was a theoretical issue, you'd have a point. But, again, there isn't any disagreement on this issue in the populace of any state. 50 states agree marriage means a man and a woman. Heck, all the history of Western civilization says the same. Why should an amendment leave this matter open for debate, any more than it should leave open the definition of "dog"?
Additionally, there is the complication of the "full faith and credit" clause of the Constitution, which suggests (with considerable precident) that a legal marriage in any state must be recognized in ALL states.