No, Congress can simply legislate against it. They have all the legitimate authority to do so that they need, since the destruction of the most fundamental unit of self-government violates every single expressed purpose of our Constitution, most notably its crowning purpose: “to secure the Blessings of Liberty to Posterity.”
But if you want to pass an amendment for the purposes of clarification, fine. Do it.
No, they can't. The Constitution makes no mention of marriage. The federal government is a government of limited and enumerated powers. That is why you need a constitutional amendment. Not for clarification purposes, but to avoid trampling on the document that you claim to be protecting. We needed a constitutional amendment for alcohol prohibition. I'm sure the prohibitionists would have argued that the destruction of the populace through alcohol abuse justifies an end-run around the constitution. I'm unconvinced. I do see homosexual marriage as a more serious threat to the union than alcohol use, obviously, but I don't support exceptions to the rules. The Tenth Amendment has to mean something.
Where have I heard this argument before...?
Ah, yes:
"The purpose of the Constitution is to 'secure the general welfare' -- therefore, welfare programs are obviously a proper activity for the Federal government."