Some people think gay sex causes aids. Why is that?

Early stereotype. In the early 1980s, not much was known about the virus, only that promiscuous homosexual men tended to contract it. This was likely a combination of high-risk behaviors (unprotected sex, multiple partners, drug use...). Recently, heterosexual transmission and intravenous drug use surpassed this transmission, but some resurgence as new generation not taking precautions. Be safe.