Quoting from this site:
A man's sexual orientation may be determined by conditions in the womb, according to a study.
Previous research had revealed the more older brothers a boy has, the more likely he is to be gay, but the reason for this phenomenon was unknown…
Writing in the journal [Proceedings of the National Academy of Sciences] Professor Bogaert said:
“If rearing or social factors associated with older male siblings underlies the fraternal birth-order effect [the link between the number of older brothers and male homosexuality], then the number of non-biological older brothers should predict men's sexual orientation, but they do not.
“These results support a prenatal origin to sexual orientation development in men.”…
Andy Forrest, a spokesman for gay rights group Stonewall, commenting on this and other studies, said: “Increasingly, credible evidence appears to indicate that being gay is genetically determined rather than being a so-called lifestyle choice. “It adds further weight to the argument that lesbian and gay people should be treated equally in society and not discriminated against for something that's just as inherent as skin colour.”
So there.
