Attended a catholic private school growing up in an extremely conservative+religious part of the midwest, not really interested in divulging anything more personal than that on the subject. But I can confirm it's not a pure myth.
Instead, here's a reminder that being prejudiced is quite normal. It's easier to associate gay people with promiscuity, to connect those dots, than to think of people differently along so many axioms of different traits.
I'm reminding myself as well. Can't judge another with one finger without the remaining fingers pointed at myself (well except for the thumb).
But what's also true is that gay people on average are much more promiscuous and much more likely to have anal sex. This is obvious to anyone who knows any significant number of gay people. And it is also reflected in countless surveys.