I have no choice about being straight, because the thought of gay sex nauseates me. I doubt that gays feel any differently about denying their sexuality.
I see three main reasons that people are anti-gay. First is that they aren't able to get beyond their own disgust, so they label anything different from their view as sick. Second would be those who are not confident in their sexuality and fear that they might be "deviant" and want to eliminate the temptation. Third are those who have been taught intolerance and justify it with their religous views.
I don't see how the government belongs in this debate. There are plenty of deviant hetero marriages that have nothing to do with procreation, so why are they legal? If two people want to make a legally binding contract, why should the government stop it?
One of the big anti-gay arguments is the very publicized promiscuity, and the inherent public health risks. Doesn't an exclusive relationship negate that stereotype? While marriage does not guarantee monogamy, it allows legal recourse and can be one more factor in discouraging promiscuity, thereby promoting public health.
I agree with something Vrai said. A marriage before god is a covenenant. Whether or not marriage is based in religion, it is no longer a religious institution. Look at the divorce rate and you can see that.