Bisexuals are harder to study but most of the examples I've come across feature (in men) a masculine brain structure with some form of different libido. On the other hand it could be that sexuality develops into a gradient so most adults may be bisexual to some degree.

I want everyone to remember that there will always be exceptions to the rule. Bisexuals appear to be more exceptional than say homosexuals with the brain structure of the opposite sex.

It makes sense that female brains produce female hormones and male brains produce male hormones. Perhaps exceptions to the rule are people with the 'right' brains who just happen to be attracted to the same sex.

Just remember no matter what causes homosexuality directly, the studies show that most homosexuals do have a brain resemblance to the opposite sex.