Homosexuality is caused by a behavior altering parasite, it is a disease. Just like rabies, toxoplasma, and dozens of other behavior altering
parasites.In this case the main vector is male to male sex. Since prehistoric times our ancestors noticed that male homosexuality resulted in the spread of the disease within their communties. As this was a direct threat to their survival in terms of the tribe's procreation rate they established a deeply ingrained cultural resistance to male homosexuality. Homosexuals were ostracized, ridiculed, exiled, and even punished with death. In modern terms it was a form of social distancing to prevent infection and control the spread of the disease. As girls were not direct vectors of the parasite lesbianism was not considered a threat, and therefore it was much more highly tolerated.
