According to Dr. Dick Swaab (neurologist) the size of the hypothalamus is a very strong indicator of sexuaml orientation. Large hypothalamus = gay, small hypothalamus = straight. He based his findings on the Dutch brain-bank ... though he does warn that, with only some 3000 brains available and with records of sexual preference not being complete, he only had several hundreds of brains to work with. Still, if he's right, that infecting someone with gay-ness is as impossible as infecting someone with right-handedness.
His findings are easily debunked, if we look at the rate of sexual preference among those raised by homosexual couples. If those children are disproportionatly gay ... then out goes Swaab's argument. And in Europe, gay couples have been raising (adopted and/or artificially sired) children for long enough that we have adults to evaluate. Among those, the rate of homosexuality is no different from that of people raised by heterosexual parents. Children are not turned gay by being raised by gays.