The Center for American Progress has a nice rebuttal of the President’s recent speech about gay marriage. It’s all worth reading, but just to give you a taste:
The American people have spoken clearly on this issue through their elected representatives and at the ballot box.
FACT: They have indeed. Eighteen states have enacted their own constitutional amendments banning same-sex marriage, and additional states are considering them. So why do we need a federal amendment?
(Incidentally, the answer to that hypothetical question is “The mid-term elections”)