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."
Times have certainly changed.
The link below discusses HIV rates among various groups. In summary, homosexual men and drug users have very high rates that has more to do with the deterioration of their health from their lifestyles. Promiscuity among healthy heterosexuals leads to low HIV rates.
An added note, if you meet an infected individual and stay monogamous thereafter you still have a very high risk of catching HIV.