What do I think? I’m frankly skeptical. For one thing, to keep the male brain from masculinizing during prenatal or neonatal development, it isn't enough to have a ‘female cell’ somewhere in the male body. The cell must be positioned at a critical point on this developmental pathway. Otherwise, the brain will develop normally.
But I have a more basic objection: chimerism is nothing new. So the human organism has had eons of evolutionary time to adjust. We come back to Greg Cochran’s point: natural selection tends to remove any condition that seriously hinders reproduction. And ‘tends to’ is an understatement. If an organism cannot reproduce, its characteristics will not survive into the next generation. Game over.
Finally, if the chimerism theory is true, exclusive homosexuality should be more common in populations with a higher incidence of twinning, such as sub-Saharan Africans. In southwest Nigeria, twin births are 3 to 5 times more common than in Europe (Akinboro et al., 2008). Presumably, there would also be proportionately more ‘phantom twins’, i.e., those that die in the womb. Is exclusive homosexuality likewise 3 to 5 times more common in Nigeria?
The post’s comments brought up another point: there is nothing evolutionarily puzzling about male homosexuality in and of itself:
I'm still not convinced that human homosexual activity is anything that needs to be 'explained' any more than whistling or square dancing or checkers need to be explained.When you strip away the family values or evolutionary or gay rights hysteria, homosexual activity is just the co-occurrence of two traits that are separate but widely dispersed in humanity:
a) enjoyment of recreational sex
b) emotional bonding with members of the same sex.
The evolutionary puzzle lies elsewhere, as another commenter pointed out (while citing a previous comment):
I don't think that the issue is homosexual behavior as much as exclusive homosexual orientation. --Glaivester
Yes, it's not so much the homophilia, but the heterophobia. Or, as I once saw scribbled on a university (men's) bathroom stall, "I'd rather die than go to bed with a woman." I doubt this was John the Baptist speaking.
It is thus exclusive homosexuality, and not homosexuality per se, that needs explaining. Facultative homosexuality does not preclude reproduction, nor does it preclude normal heterosexual development of the male brain. It may arise simply because access to women is limited (as in prisons or in polygynous societies where older men monopolize the pool of fertile women). It may also arise in hypersexual men who have low thresholds for sexual excitement, i.e., who are turned on by anyone or anything that remotely resembles a woman.
Akinboro, A., Azeez, M.A., and Bakare, A.A. (2008). Frequency of twinning in southwest Nigeria, Indian Journal of Human Genetics, 14, 41-47.