Hi, first post here, so please excuse my new-poster nervousness

I've been very interested to read everyone's thoughts on the origins of sexuality; this is an area I'm heavily interested in both personally and professionally (as a dreaded psychology major), and I've done a little theorising based on my main background and interest in evolutionary psychology and gender interaction. I guess the main issue here is not so much the origin of homo-/bi- sexuality, but rather sexuality itself. If we follow evolution, it initially makes the most sense to assume that everyone would be heterosexual - after all, every human being on the planet is the product of generations upon generations of heterosexual mating, and in theory will have inherited biological mechanisms that promote identification of and attraction/desire towards members of the opposite sex. Of course, this isn't the case; but I find the 'gay gene' explanation rather unsatisfying, as well as a little contradictory to what natural selection would suggest - if a gene or genes exist whose sole purpose is to produce homosexuality, they'd have been pretty much wiped out within a few generations due to the at worst, biological impossibilities and at best, biological complexities of same-sex couples producing offspring - and even discounting the innate desire to have children, the fecundity of gay men is far, far, below their heterosexual counterparts and not enough to keep an exclusively 'gay gene' at its relatively significant prevalence level. Of course, discounting this leaves only the similarly deeply unsatisfying 'lifestyle choice' option, which is so inherently ridiculous I don't feel the need to explain why it's garbage.
So, instead of the existence of a 'gay gene', I'd argue that homosexuality arises as an equal counterpart through evolutionary mechanisms that promote heterosexual fitness. Many, many studies have shown repeatedly that homosexuality in men shows strong correlates to femininity, especially traits like sensitivity and empathy, which interestingly are traits that are consistently rated highly desirable by females (other studies have more shakily shown that gay men show increased kindness, altruism and are less prone to engage in violent crime, although these are much less conclusive and established). Female preference for more feminine traits in men cannot be underestimated. Imagine a scenario in which a woman has a choice between two males - both attractive, both heterosexual. However, one is highly masculine and displays many typically male behaviours, like pursuing many sexual opportunites at once and having high levels of aggression, while the other is still masculine, but shows less of these traits plus a few more feminine ones (kindness, loyalty). For the female, it makes the best bet to take the more sensitive male, who's less likely to desert her, and has qualities that make him a better father, increasing the survival chances of her children. Therefore, whatever alleles caused the slightly more feminine behaviour get passed on to the next generation. If this happens on a wide scale, over a few generations the amount of 'feminine behaviour alleles' (FBAs) in the gene pool will start increasing rapidly. Now, FBAs may somehow alter the male brain, pushing it slightly in a female direction - something borne out by studies with neuroimaging that show quite a few similarities between brain regions of gay men and straight women versus straight men and gay women.
If there are numerous FBAs in the gene pool, a male stands a chance of inheriting any number of them from his parents. Using a hypothetical 10-allele scale, we could suggest that getting 2 or 3 FBAs feminises a man's brain slightly, producing a straight offspring with some female traits that will improve his later reproductive success. However, a son who inherits 6 FBAs out of ten could experience a larger feminising influence, whereby areas of the brain affecting sexual orientation are shaped slightly, perhaps producing a bisexual man. 8 FBAs could completely shape the orientation region towards the female side and produce a gay son (but one who still has a solid and comfortable male gender identity), while getting the full ten FBAs could completely feminise the brain, perhaps producing a male who would be transgendered.
Of course, this is all just theory, but I think it would help the research community to stop looking for simple 'gay genes' and start looking a little deeper into sexuality and gender as a spectrum. I've tried to account for bisexuality and transgenderism too, but I know a lot of assumptions are required to make this work (and of course, environment plays a role too, as in discrepancies between MZ twins).
What do you guys think? I know I've only discussed males, but I can do so for females too if anyone is interested. Also a quick apology for a lack of further depth/citations, but this is supposed to be a fun break from my long name-and-date filled dissertation.
