Increased understanding and education is probably another factor.

The emerging concept of HIV transmission is that, for efficient transmission, the virus does not pass readily through intact skin but has to be "worked into" the tissue in such a way that it gets to its target cells for infection which are below the sin. Sex by nature involves friction and friction helps to work the virus into place to cause infection. If exposed to HIV, the probability of infection therefore varies with, among other things, the thickness of the skin involved (genital skin is thinner than the skin of many other parts of the body and the skin on the palms and soles are the thickest of all), and the amount of friction involved (thus there is more friction for rectal intercourse than for vaginal intercourse and more friction for vaginal intercourse than oral intercourse). These principles in turn are part of the reason that masturbation is always safe sex.

A man who has sex with men, for instance, could use these principles to inform the decision to partake in sex in a particular type of way, but not another

As for frottage. most frottage is really just a form of masturbation and therefore no risk.

Whereas for anal, that's the highest risk.