All very interesting - I wasn't aware of the American findings. However it still comes down to a lacking hormone during development and that widdle area of the brain which was confirmed by the studies in the 90's.
Either way, biases against homosexuality within social groupings are more to do with our natural instinct to cast out that which is considered "unhealthy" in order to keep the whole strong and pump out the offspring as a great percentage of births were destructive to both mothers and their children.
It is only in the last century that man has had the luxury of true leisure time, including the luxury of being able to express an alternative sexuality without endangering the population of the tribe. Heck, there's now too dang many of us on earth, we should be encouraging those with alternative lifestyles.
Cheers