Sid Schwab hits the nail on the head.
Gay rights are civil rights. The only arguments against gay marriage and other rights are religious ones, based on a book full of contradictions on much more serious matters than marriage. Killing your kids, for one. To believe gays should be discriminated against, you have to believe that homosexuality is a choice, and that it's religiously prohibited. The first is clearly scientifically wrong (what??? You mean right-wingers reject facts to maintain their deep-seated prejudices and belief in pre-failed policies??); the second has no business in civil law. If you believe in the Constitution (you know, like all those teabaggers) then you can't reconcile the idea of states voting away the civil rights of a class of citizens based on sexual preference any more than you can do so based on skin color. Or gender. And, yes, I'm well aware that there are plenty of right-wingers who'd happily do both.
I don't agree with everything he says in this paragraph, but it's true that religious prohibitions have absolutely no place in this country's making of law. We are ruled by the Constitution, not the Bible or any other religious text. If you read the Constitution (and I have, thanks to the little booklet sent me by the ACLU) you'll know that religion is not mentioned, and the Ten Commandments are nowhere to be found.
I'm a person of faith, but it's also clear to me that atheists and agnostics can come up with just as good a statement of ethics and human living as any Judeo-Christian, and they can certainly write just as good a governing document for a country. (Of course, there's that little pesky thing about "three-fifths of a person," but the beauty of the Constitution is that as society evolves, and decides that discrimination against people of color and women is no longer acceptable, the principles of the Constitution can evolve along with this--at least for those who acknowledge that it is indeed a living, breathing document, and not an ossified, rigid, unchangeable relic of a group of people long dead.)
At any rate, read the whole thing.