The major argument against gay marriage by the "Moral Majority" in this country is that homosexuality isn't something that a person is born into.  They argue that a person isn't born gay, but instead choose to be gay later in life.

They say that since it is a choice, and not a birthright, gay people cannot be discriminated against.

We choose our religion, our profession, our partner - and since all marriage in this country is a choice, why should it matter (regardless of orientation) who someone chooses to marry?

"Nature v. Nurture" is a moot point.  It doesn't matter if a person was born gay or if it was their choice - their rights are still (supposed to be) protected under our Constitution.

Also, the last time I checked, morality wasn't a legal standard.  If you don't agree with homosexuality, that's fine, but don't deprive anyone of their rights based on your own opinion.