Be forewarned, I'm going way out there in this post. My beliefs about sexual orientation are bizarre even among the unorthodox. (Well, at least among unorthodox North American Christians!) My theory is uncommon but not entirely original; there are variations of this thought among those who research and believe in reincarnation.
We find very few clear descriptions of the world to come in the Bible but on one occasion, when the Sadducees present Jesus with a hypothetical situation involving marriage after the resurrection, Jesus shares some surprising information:
"At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven." (Matt 22:30)
Now, I have no idea what the angels in heaven are like in form or character but I'm going to make a few fun guesses. What if there is no marriage in heaven because there are no sexes? What if the angels, and our heavenly bodies, are androgynous? (I am aware that angels showed up as men in the OT but I don't think that is their true form; just one that made for good communication!)
Could homosexuality be an overstatement -i.e. a case of the pendulum swinging too far in the opposite direction as the soul struggles to overcome the male/female polarity? Or perhaps when a soul has incarnated as the same gender for many lifetimes it carries with it a sexual imprint, subconscious memories, making it difficult to identify with their new gender.
I'm not discounting the biological component here: sexual identity/orientation most definitely has a physical component, but what if the soul is the architect, the mind is the builder and the body is the temple? If that is the case then being straight, gay, bi or whatever isn't just an issue of anatomy/physiology, psychology or sociology; it runs even deeper than the typically understood influences of nature and nurture.
The irony here is that it seems I've come to the understanding that at our deepest level of being homosexuality (and all non-heterosexual sexual identity expressions) still 'miss the mark.' True, but I don't believe missing the mark automatically translates as sin. Let me illustrate: suppose two dark-skinned/dark-haired people living in a very hot climate give birth to a child who has considerably lighter skin and hair. From an evolutionary perspective the child has 'missed the mark' because the recessive fair-skinned/fair-haired genes do not offer as much protection from the sun. Likewise, non-heterosexuals are at an evolutionary disadvantage because they are less likely to procreate. From a purely physical perspective this is not ideal. However, from a spiritual viewpoint we can see that the overstatement, or the state of transition from one gender to another, is a necessary experience for the development of that particular soul.
In the end, when the human condition is no longer a necessary lesson, there will be no need to identify as gay or straight, male or female because we will all be reconciled as one. At least, that's what I believe.