A short excerpt from the SciAm link above:

"Researchers using brain scans have found new evidence that biology—and not environment—is at the core of sexual orientation. Scientists at the Stockholm Brain Institute in Sweden report in the Proceedings of the National Academy of Sciences USA that gay men and straight women share similar traits—most notably in the size of their brains and the activity of the amygdala—an area of the brain tied to emotion, anxiety and aggression. The same is true for heterosexual men and lesbians.

Study author, neurologist Ivanka Savic–Berglund, says such characteristics would develop in the womb or in early infancy, meaning that psychological or environmental factors played little or no role.

"This is yet another in a long series of observations showing there's a biological reason for sexual orientation," says Dean Hamer, a molecular biologist at the National Institutes of Health (NIH), who was not involved in the study. "It's not just a reflection of people's behavior, nor is it a choice, nor is it something in their rearing environment. [The study] shows that it's something that people are born with.""