I'm reading a collection of essays called Queer Theology at the minute, which may well make further appearances as it's really interesting, and means that I get to talk about sex (woo!). James Alison is a Catholic theologian, looking for a Catholic response to the issue of gayness. Personally, I'm a bit irritated by his generalisations about Catholic and Protestant theology, but they lead him to interesting thoughts, so we'll go with it.
Alison argues that there are two ways we can see humans and their desires. The first he identifies as a traditional Protestant view, which is that all humans are sinful right through, in every part, so much so that what they need from salvation is to scrub out the whole of their pasts and start again from scratch. Nothing of who they were before salvation can be redeemed. Whether or not anyone has taken this extreme position (and I'm probably caricaturing a little myself), it's definitely a tendency that you see in various theologians/theological traditions. The alternative is to see humans and their desires as essentially good, albeit distorted by sin. This view sees sin less as a virus which hopelessly contaminates everything it touches, and more as basically good intentions directed to the wrong ends - sin as missing the mark. In this view, salvation isn't about scrapping everything and beginning again, but about slowly untangling and unbending our muddled up desires, reorientating them towards right ends and thus enabling us to flourish.
In the first view, because the world is so screwed up, we can't really look to it to find out what is right - we just have to trust the commandments we find in the Bible. In the second view, the world reflects, albeit in a somewhat distorted way, the nature and intentions of God, and so we can learn from it: if things we thought were commandments seem to inhibit human flourishing, we can reassess whether they really are commandments, and whether we've been reading them right.
Within the second view, there are two possibilities for understanding homosexuality. We can see it as being basically wrongly directed (so the fact that our desire is directed to people of the same sex is a sign of its brokenness), or we can see it as fine, but distorted by sin in the same way that everyone's sexual desire is (so it's fine to fancy people of the same sex, but sin means that good desire is a bit twisted and so tends towards selfishness, jealousy, cruelty etc). Is it the object of desire itself or just the way the desire works out in we broken humans that's the problem? Is homosexuality redeemable, or is it "objectively disordered" like, say, kleptomania? We have studied kleptomania and have come to see where it comes from, what good desires it's a distortion of, and how people can be helped out of it into lives of greater flourishing. Alison argues that, if the Church is to continue to assert that homosexuality is simple "a severely defective form of heterosexuality" then it needs to "produce regular and sustained witnesses to heterosexual flourishing emerging without violence from the life stories of people who had assumed they were gay on something like the same scale as there are regular and sustained witnesses to gay and lesbian flourishing emerging without violence from the life stories of people who had been taught that they were heterosexual." Ouch.