Assuming what you say is true, what prevented it from taking place 30 years ago, at the start of the HIV pandemic? I came of age (puberty) in the 1970s. Everything was sex, Sex, SEX! It was common to see advertisements for massage parlors in "entertainment" papers throughout Madison, WI. There was at least one well advertised massage parlor right next to the state capital building - with men in business suits coming-and-going (pun intended). From the late 1960s to early 1970s, Madison had "The Blue Bus". It provided infectious disease "curb-service". All one had to do was put on a good disguise, wait on one of the designated street corners, and hop-on The Blue Bus. By the time I got to college, it had stopped running. But the jokes about it lingered for years. The point of this paragraph is that there was never a shortage of unprotected heterosexual activity at the start of the HIV pandemic. Either something acccelerated the spread of HIV in gay men (contaminated vaccine, I.V. drug use, etc) OR something delayed the introduction of HIV into heterosexuals, OR there is a valid explanation for the (more than) 30 year delay of a HIV into heterosexuals.