""Research suggests that the homosexual orientation is in place very early in the life cycle, possibly even before birth. It is found in about ten percent of the population, a figure which is surprisingly constant across cultures, irrespective of the different moral values and standards of a particular culture." Statement on Homosexuality, American Psychological Association, 1994-JUL."
http://www.guardian.co.uk/science/2008/jun/16/neuroscience.psychology
That is a link to a sceintific showing that gay men and straight women have similarly shaped brains.
From that study: "Striking similarities between the brains of gay men and straight women have been discovered by neuroscientists, offering fresh evidence that sexual orientation is hardwired into our neural circuitry.
Scans reveal homosexual men and heterosexual women have symmetrical brains, with the right and left hemispheres almost exactly the same size. Conversely, lesbians and straight men have asymmetrical brains, with the right hemisphere significantly larger than the left.
Scientists at the prestigious Stockholm Brain Institute in Sweden also found certain brain circuits linked to emotional responses were the same in gay men and straight women.
The findings, published tomorrow in the US journal Proceedings of the National Academy of Sciences, suggest the biological factors that influence sexual orientation - such as exposure to testosterone in the womb - may also shape the brain's anatomy."
And our own resident neuro-scientist, Thorn has said that the structure of the brain of many gay men is different than straight men.