What is the difference between male and female brains? How does sexual orientation affect those differences?
The anterior hypothalamus is the part of the brain that helps govern sexual behavior that is more typical for males. For example, men tend to have a preference for the look, feel, and smell of women rather than men. The anterior hypothalamus has been shown to play a part in this.
Anterior hypothalamus: larger = more sexually attracted to women?
An NCBI study found that the anterior hypothalamus is 100%+ larger in straight men compared with gay men.
The size of the anterior hypothalamus in gay men is actually more similar to women's.
Another study found the anterior hypothalamus is smaller in transsexuals who became female (and would thus be more sexually attracted to men) while it is larger in transsexuals who became male (and would thus be more sexually attracted to women).
These studies would indicate that there is a correlation between the size of a person's anterior hypothalamus and the extent to which the person was sexually attracted to women.
Those attracted to women (straight men, transsexuals who became male) had larger anterior hypothalamus's. Those attracted to men (gay men, straight women, transsexuals who became female) had smaller anterior hypothalamus's.
These findings could indicate a genetic foundation for sexual orientation, beginning in the brain.
The aforementioned studies did have small sampling groups, and more research will need to be done before any conclusive findings can be drawn.
But they do indicate potentially promising findings.