I'm still feeling a little drained, and I'm not sure I have a ton to say about the subject, but here goes.
Contrary to what some people have argued, I have no doubt whatsoever that the words of the Bible explicitly disapprove of homosexual relations -- and Judaism and Christianity are clearly not the only religions that view homosexuality as sinful in nature.
However, the topic of gay marriage is not properly an issue of religion, but an issue of state. (Perhaps these two forums -- state and religion -- are not ultimately so easily divorced, as the laws must appeal to, in the end, some conception of the good, which, I think, must be transcendent if it is to mean anything. However, there is supposed to be a marked separation of church and state, and, in any case, we are not discussing these subjects on the meta-level at which they converge.)
In the eyes of the government, then, people are not to be discriminated against with respect to their sex -- which is to say that the sex of a person should not make any difference in the way that a person is treated. Given that, the government has no right to disallow two people to marry on the grounds that they are not persons of opposite sexes, because their sex should not matter, nor should it affect what rights they have. And if, as some have argued, marriage is a sacred and religious institution, and the government has no right to tell the church how to define marriage, then marriage should be kept out of the sphere of government. But insofar as the government recognizes such unions, to deny two people the right to come together and to be afforded the benefits of the government in this -- simply because of their sex -- is to discriminate against them on the basis of their sex, which is clearly prohibited. As far as the state is concerned, all partnerships made official should be "civil unions"; let the spouses call themselves husband and wife (etc.) in the church, or behind the doors of their homes. This would, no doubt, require much reform, but if religious persons are so interested in "protecting" the sacred institution of marriage, they should be more than willing to undertake it. Why would you leave your hallowed artifacts in the hands of politicians and judges? Consign them to your houses of worship, and your hearts, where they belong.
Insofar as we live in a country in which church and state are supposed to be separated, I would submit the above arguments even if I believed that homosexuality was truly wrong. I do not believe that, however. Many will argue that to "be" homosexual is a choice -- in response, I challenge these people to pinpoint the exact moment that they made a conscious choice to "be" heterosexual. And if they conclude that they had no choice in the matter, why should they believe that homosexuals do? (Admittedly, this issue is complicated by the fact that there do seem to be some people who choose to pursue members of the same sex -- "experimentation" by college students being one example. But this is no basis for claiming that all persons attracted to same sex partners exercise such autonomy.)
Yet many will argue that to sanction homosexuality (by allowing homosexuals marriage and its subsequent benefits) is to support a certain kind of lifestyle -- namely, a promiscuous one. (Insofar as the government is unwilling to take a definite legal stance against promiscuity and to begin to prosecute all persons who engage in illicit sexual practices, homosexual or otherwise, this is a moot point, but I'll say more about it.) And while there may be many homosexuals who do adhere to that kind of lifestyle, I do not think that being attracted to members of the same sex necessitates promiscuity any more than being attracted to members of the opposite sex does. Moreover, I have no doubt that there are many homosexual persons involved in monogamous relationships and who would be faithful and devoted spouses, just as I have no doubt that there are many heterosexual persons who delight in sexual conquest and who betray their spouses on a regular basis. Some say that to allow persons of the same sex to marry would cheapen the institution of marriage; I say, on the contrary, there is an excess of heterosexual partners who do that just fine, and there are a number of homosexual lovers who would beautifully exemplify the bond between two persons that marriage represents.
But finally, I do not believe that people are ultimately defined as individuals by their sex, any more than they are defined by their shoe size, their height, the color of their skin, or any physical characteristic over which they had no control. Though society may place more/less emphasis on these, in the end, none of them things matter so much as the character of the individual -- or, more properly, the soul. (If that's too hokey for you, insert any word which you think is appropriate to refer to the transcendent essence of an individual.) And as I do not think the soul has sex as we know it -- any more than it possesses a shoe size, height, skin color, etc. -- I do not see how the biological sex of a person can or should prevent one individual from coming to know and love the soul of another.