On the other end of the spectrum, conservative Christians (at least the ones educated enough to understand that it's not a matter of choice) still generally hold to the idea that same-sex attractions are entirely psychological, most commonly through Elizabeth Moberly's theory of the "defensive detachment."
For my part I can fit my early life experiences into Moberly's model (poor relationship with same-sex parent) well enough that I accepted it without question when I first read it. But even aside from the fact that there are millions of people who don't conform to that stereotype - both gay individuals who had good relationships with their same-sex parent as children and straight people who didn't - there are things that no psychological theory can adequately account for.
Of course, the stock response to such criticism is that it's all in the child's perceptions, and that therefore a boy who's beaten daily by his father can still turn out straight, while one who's deeply loved and affirmed by his dad can still turn out gay, all because of some subconscious misunderstanding on the child's part. Which, of course, can conveniently never be disproven since it's pure speculation and completely untestable.
This utter disregard for the scientific method is one of the reasons that the APA and its sister organizations pay little attention to Moberly's theory and its variants, Exodus and NARTH's cries of "politics" notwithstanding. ("Hello, kettle? This is the pot. You're black.")
It is plausible that same-sex attractions are purely psychological for some individuals. Reparative therapy does reportedly work for a subset of those that have sexual abuse in their backgrounds. For the rest of us? Not so much. And if Moberly's theory was true for everybody (or even the majority), one would expect her solution to work for more than a tiny number of people.
But that still leaves questions unanswered. If I'm gay because I patterned myself after my mother instead of my father, why didn't I develop more effeminate mannerisms? Why have I never had the slightest desire to dress like (much less to actually be) a woman, if I wanted to be just like mom? And could that patterning really be adequate to explain why I've been largely non-aggressive from birth?
Interestingly enough, NARTH has provided an answer to these questions, though not all of its members would endorse my application of their own evidence. In the article Gender Differences Are Real by Frank York, Dr. York refutes the notion promoted by some radical feminists that the differences between male and female are purely societal constructs by arguing first from genetics (the most obvious difference) and then from hormonal differences. He demonstrates how gender-specific behaviors are determined by hormones – testosterone and other androgens in males, estrogen in females – as illustrated by the following example:
At the University of Wisconsin, researchers injected testosterone into unborn female monkeys. Monkeys engage in very sex-stereotyped behavior, according to Stossel; the males are aggressive and fight, while the female monkeys typically groom and nurture the young. When the testosterone-injected females were born, they didn't groom or nurture their children. They fought and behaved like males.
Although the article pays only passing lip service to the ramifications of this point, the conclusion is clear: if gender-normal behavior is dictated by prenatal hormone levels, then gender-abnormal behavior (effeminacy and lack of aggression in males, tomboyish behavior in females) logically has its roots in the same source.
It's a complicated line of study that scientists will no doubt need years to investigate fully, but it shows promise. Most perplexing, perhaps, is the fact that, while hormones have a direct impact on physical development as well as mental (as evidenced by the effects of Androgen Insensitivity Syndrome), most homosexuals are largely indistinguishable from their heterosexual counterparts in physical terms (with the possible exception of brain development).
Some ex-gay proponents have begun to acknowledge the likely role of hormones in the development of sexual orientation, though they would still argue that hormonal (and/or genetic) influences create, at most, an increased propensity toward homosexuality that is still fully dependent on a psychological element (and therefore curable). Whether or not they can come up with a testable theory that's worthy of mainstream consideration remains to be seen.
Ultimately, of course, the question of how people become homosexually oriented is a peripheral issue (albeit a very interesting one). Conservative Christians would continue to regard all gay relationships as unconditionally wrong even if a purely 'natural' (i.e. genetic) cause was found, just as you wouldn't see gay couples splitting up in droves to pursue celibate lifestyles if their attractions were proven to be purely psychological. And both sides would have a valid argument for maintaining their respective positions. In the end, the morality of acting on one's attractions is not dependent on how one came to have those attractions.
Despite that, the question of origin remains a muddy one. It's simple human nature to reach one's conclusions first and then work backwards to tailor the evidence to fit those conclusions, and people on both sides of the aisle are equally likely to do so. Christians, of course, are inclined to defend their bias by arguing that they're working backwards from God's conclusions, but in the end it's just the same old eisegesis in fancier packaging.
Someday, maybe, the picture will become clearer...