Q. I enjoy your Q&As, but I’m wondering why you haven’t just come out and said what causes a brain to not be heterosexual? It seems as if you’re dancing around that issue.
A. I would be more than willing to report on specific research that pinpoints an exact cause for “a brain to not be heterosexual” if I knew of any. Scientists simply do not clearly understand the origins of individuals not being “straight.” Wikipedia actually put it very well by pointing out that no simple, single cause for sexual orientation has been conclusively demonstrated, although research suggests that it is by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment.
The Netherlands Institutes for Neuroscience released an abstract containing this information: During the intrauterine period the fetal brain develops in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed into our brain structures when we are still in the womb. However, since sexual differentiation of the genitals takes place in the first two months of pregnancy and sexual differentiation of the brain starts in the second half of pregnancy, these two processes can be influenced independently, which may result in transsexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinization of the genitals may not reflect the degree of masculinization of the brain. There is no proof that social environment after birth has an effect on gender identity or innate sexual orientation. [Source]
My opinion is that the sexual templating of the brain is an extremely complex process. And even if the presence of a gene is validated, there will likely be co-factors that play into how the brain actually templates. About all one can say at this point is, "stay tuned."