Homosexuality is within the sexuality range of humans -- always has been, and always will be. It is the same as there being some people who are more adventuresome, others excel in music, still others are brilliant mathematicians, and yet others become great leaders. Humans evolved to live within tribes, and each person within that tribe had functions essential to the success of that tribe. Even homosexuality had a purpose in such a way of life. It has been documented in other species which live in extended family groups that there are often non-reproductive members which stay and help support the raising of their biological relatives. Wolf packs, for instance, typically form around one breeding pair and offspring and/or siblings of that pair remaining to hunt together and help rear offspring of the one breeding pair, while not breeding themselves. Do we search out a "cause" for those non-reproducing individuals being born "the way they are"? No, because the system as it is just plain works.
There is some scant evidence that somewhat more gay men are the youngest of multiple sons born to the same mother, but this is only represented as a statistic that's slightly off from average. It doesn't imply that all youngest sons are gay, nor that all oldest sons are straight (I'm the only son, first born, and only gay-identified member in my entire extended family). The thought behind this hypothesis is that it is more advantageous to a woman's mitochondrial DNA (which is inherited solely from the mother, so sons do not pass on their mother's mDNA) to have daughters and non-reproductive sons. Why? Because their daughters will pass on their mDNA to their grandchildren, but their sons will bring forth grandchildren carrying another woman's mDNA. It's possible that after one or more male-carrying pregnancies, some women effect something during pregnancy which decreases the liklihood of future male offspring having a desire to reproduce, i.e. be attracted to the opposite sex. But as I was prefacing earlier, there is no concrete evidence, and the resulting data is only slightly skewed from the expected norm, so even if there IS a push resulting from this, it would be extremely minor when taken into consideration along with all the other variables involved in forming a human's adult personality and behavior.
Point of my post -- stop trying to find a "reason" for homosexuality (or bisexuality or gender identity), because behind the search for a "reason" is a search for a "cure." And I'm doing just fine as I am.
