For me, it's a little bit complicated.
The Christian-- or, at least, the "Catholic"-- explanation for prohibiting homosexuality seems similar to its explanation for prohibiting birth control. And, like all good Catholic teaching, there seems to be a healthy dose of Aristotelian thinking involved.
Here's the shortened and probably oversimplified version: Sex has a three-sided "purpose" for people, which is intimacy, pleasure and procreation. Any kind of sex that doesn't fully realize the "purpose" of sex-- like sex not open to procreation (using birth control, sex with people of the same gender, casual sex, etc) is missing the point. Or, "sinning," as the word "sin" is defined most accurately as "missing the mark."
Honestly, it's one of the aspects of Christianity I have the most trouble with. I'm straight as an arrow, so it doesn't really matter much to me on that level, but it is a hard trying to explain that position to homosexual friends. Moreover, in Jesus' time, people didn't necessarily have to think about issues like global overpopulation. Most churches agree that good sex is absolutely essential to healthy, married relationships-- and yet so not having too many children, both for the sake of the family and the earth. I really just don't understand what the problem with two responsible, committed people having "protected" sex is.
But as far as homosexuality goes, I'm also thinking that in Jesus' time there weren't many homosexuals having sex for the sake of expressing mutual love. In those days, a good portion of people in Rome and so on were bisexuals who had families but spent a lot of their time in bathhouses and dens drinking, eating, and boning other dudes. The fact is, the Bible only talks about homosexuality as it appears as a matter of lust, and, as far as I know, never talks about the natural genetic "phenomenon" of homosexuality, or homosexuals who are looking for mutual, exclusive, loving relationships with other people who share that "phenomenon."
To be honest, as much as it goes against Church teachings, and is much as it may be against Scriptural teachings, I'd say that most good
straight Christians fall way short of the Christian ideal when it comes to sex, whether it be through pursuing one-night stands, using birth control, "pulling out," viewing pornography, and so on. For that small percentage of Christian homosexuals who are looking to build a relationship with other homosexuals that
isn't mired in pure lust, I'd say what matters is what God puts in their hearts-- including whether they're homosexual by nature or by the same kind of desires that led those ancient Romans to screw around in the bathhouses. While there will never be a scriptural defense, scripture doesn't seem reference people who are homosexuals by some reasons other than through their own will. It just doesn't.
(I patiently await someone who actually knows their scripture to tear me in half.
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