I think that in the beginning, everyone considering this question has to do a basic double-take along the following lines:

"What? Suddenly we've come to a point in Christian life, supposedly based on love, where the deepest human love we can experience, the love for someone we naturally fall (and grow) in love with as our truly compatible life partner [a type of relationship that always includes sex], doesn't count at all and in fact is considered evil? How did this happen? When did love become a 'house divided against itself' and suddenly collapse here into a pile of rubble in the beginning of the book of Romans, to be replaced by a theoretical 'God's plan' that completely ignores this kind of deep love in the case of non-heterosexuals?"

At this point one can either salute and say, 'well, kismet, God says so and that's that' or one can take God's commitment to love very seriously and say, 'maybe I'm missing something here and the integrity of love can still be salvaged somehow.'

Most people who have tried to put love back together again have done it in the liberal christian way, which means writing off some scriptures as historical artifacts. And it's true there are a number of problematic scriptures that one would like to imagine were historical insertions or misunderstandings of some kind: a little collection would include God's proclamation of the sons of Ham (Africans) as the servants of other men, God's orders for complete holocaust-style genocide of much of Canaan, the slaughter by a divinely directed bear of a whole group of boys who were teasing Elisha the prophet about his bald head, and St. Paul's commands to slaves to obey their masters and not make trouble. For liberal Christians, the famous Romans passage about male-male sex seems to fit well in this group, since its imagery seems misguided from the get-go. If you try to apply it to homosexuals, you will find that gives a distinct portrayal of men who, after starting to worship the creature instead of the creator, "leave the natural use of women," and this portrayal doesn't seem realistic. It can't be mapped on to any experience most homosexuals have ever had. The passage can, of course, be read as an uncomprehending overview (involving purely symbolic "leaving of the use of women" rather than actual decision-based leaving of that use), but it seems curious that an all-knowing God would not say something to acknowledge the love that is being deterred, rather than just the fraction of the relationship that is based on eros. As I've always said, if there were a biblical passage that said something like, "as for sexually based same-sex relationships, we recognize that they can be very loving and beneficial but they're against God's wishes, so forget it," it would be case closed. But the scriptures show no evidence of understanding this common reality.

Well, some people can live very easily with being a liberal Christian in this way, but there is a problematic passage saying that all scripture is good for instruction. Suppose one were to be an orthodox Christian and take it seriously. It was, in fact, my orthodox-Christian quest to find out what the seemingly misguided Romans scripture was good for that ultimately led to --- the obvious! There is, in fact, a kind of same-sex sexual activity that fits the passages perfectly, and it's one that was so common in the ancient world that it probably made gay relationships as we know them today look like an obscure deviation from it (as indeed, is exactly the case today in, for example, Iran.)

Some writing about this starts at http://cblf.org/messages/53046.htm (you might want to read the starting post from bigcalv just for reference) and continues on after bigcalv answers.