Bill Nye weenies out on this issue. From a strictly evolutionary perspective (reproduction is the central idea, right?), a genetic predisposition to homosexual behavior is simple natures way of saying: Terminate this experiment.’
It’s not necessary for all individuals to procreate for a species to continue. Homosexuality is simply a failure of “normal” development. Sexual reproduction goes “wrong” an awful lot of the time.
Scientifically, it may just be an environmental reaction to any local under-population (**IF** you consider masturbation to be single-sex) or over-population event...say, sticking a straight man into a crowded prison filled only with men for 30 years.