Why? I'd say that we're a pretty enlightened bunch here and some of us are against gay marriage.

That may be, but changes to the Constitution should not be made lightly. It is the foundation of the US' entire system of government, and changing it as an election campaign gimmick or to enforce one's own beliefs before the consensus of the nation has been obtained is an extremely poor idea. You have to wonder what is next... an amendment to permanently ban trade with Cuba?