I'm surprised that nobody has brought up the Corpus Callosum. It is the bundle of nerves that connect the 2 halves of the brain. It is proven that some straight men have a corpus collosum that is nearly 2 times the size of gay men, and the reverse for women. Granted, there was a overlap of gray area...but perhaps this is where enviroment comes into play. But, as far as I know, there have been no cases where an overally small corpus callosum resulted in a straight man, or an overly large corpus callosum resulted in a gay man.
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