Let's talk (about the gay marriage debate)

I don't think I know what my position is on the gay marriage issue yet. But I did notice an interesting point in the debate. There is a common argument you hear by opponents of gay marriage that is premised on the conviction that homosexuality is immoral. You hear this argument so frequently that you naturally assume it's at least valid, and it's only when you stop and think about it that you realize it doesn't seem to hold water.

Any non-religious idea of morality that we accept today in America is at the fundamental level based on the utilitarian necessity of not causing harm to someone or something - with the caveat that one may justifiably do harm where there is conscious consent involved (I would welcome objections if anyone's got them). Applying this standard of morality to many examples of so-called sexual perversions, you would conclude that most are in fact immoral. For example, the universally detested act of bestiality would be wrong not because it is aberrant and unnatural, but rather because it consists of the mistreatment of a powerless subject - much in the same way that picking up a cat and swinging it around by its tail would be abusive and wrong. Likewise, man-boy or man-girl (man-child)? relationships are immoral because the power disparity and plain immaturity of one of the participants prevents the establishment of acceptable consent. Homosexuality is of course between two presumably consenting adults, and not even the staunchest opponent of homosexuality makes the claim that it causes harm to either one of them - if it did, wouldn't it be more logical for them, as gay haters, to advocate it? To say homosexuality is wrong because it is unnatural or improper doesn't work I think, because we as a culture don't seem to define morality on those grounds. The only other justification for calling it immoral must lay in religious teachings. I should hope that we as a country are beyond the stage of basing any government affairs, especially legislation, on religious beliefs.