Nothing in the story to back up the assertion it started with heterosexuals. Yet they can't help themselves dragging down the traditional norm.
It's quite plausible. There are far more heterosexuals than homosexuals, and even if the rates are higher with gays, the sheer numbers are likely to be higher with heterosexuals.
The relevant difference is not "straight" or "gay. The relevant difference is "chaste/monogamous" or "promiscuous".
Promiscuous heterosuxuals have no reason to think themselves exempt from moral censure or public health hazard. Promiscuous heterosexuals are not "the traditional norm."