Sexologists call this kind of phenomenon "homosexual transgenderism" and suggest it is fairly common around the world. Sometimes "homosexual transgenderism" is enacted via a humane cultural system, as in Samoa, and sometimes via a phenomenally oppressive one, as in Iran, where feminine homosexual men have been given the choice of transsexualism or death.

Regardless of the cultural system, social pressure to appear straight seems to be fairly intense cross-culturally. Indeed, one is inclined to wonder, if being straight is just natural, why does it require quite so much policing?

Still, there does seem to be some biological evidence that at least some straight people probably were born strongly inclined that way.

Ray Blanchard of the University of Toronto has articulated the "fraternal birth order effect" (FBOE): The more older brothers a male has from the same biological mother, the more likely he is to be a gay adult. The theory is that the mother builds up an accumulating immune response to male fetuses, progressively dampening down masculinity of later-born male fetuses. That's just a theoretical explanation, although the FBOE itself is unequivocally real; it holds up in study after study across cultures. Blanchard has estimated that the 15 to 29 percent of gay men are gay by virtue of the FBOE. (The effect doesn't exist with women.)

While the FBOE is usually used to talk about the origins of male homosexuality, it could just as well be seen as suggesting that a particular womb environment is likely to produce babies who will grow up to be heterosexual men. In other words, the FBOE suggests that it is likely that many straight men were born inclined to be straight. Note this wouldn't be because of these straight men having been born with a "straight gene." They would be born inclined-straight following complex interactions of maternal and fetal genes.

Is there any evidence for "straight genes," other than the rather indirect evidence of the large number of people who identify as straight? Researchers have looked at sexual orientation among monozygotic twins -- that is, twins who are genetically identical -- versus dizygotic twins. Monozygotic twins are more likely than dizygotic to have the same sexual orientation (to both be straight or both be gay), but even they are not always matched. Bailey concludes that the data are "consistent with some genetic influence" for sexual orientation but that the data are "not overwhelming." He goes so far as to say "the evidence from twin studies for innateness of sexual orientation is pretty weak."

That said, Bailey does see some other evidence for an innate component to sexual orientation, at least in males. He points to cases where genetic males have been surgically and hormonally turned into girls in infancy, either because of childhood accidents that obliterated their penises or because they were born without penises and thus doctors subjected them to sex change. As adults, these folks are typically attracted to females. Says Bailey, "if you can't make a genetic male be attracted to other males by rearing him as a girl from early in life, how likely is any socialization theory of homosexuality or heterosexuality? I think not likely," at least for males.