simple solution for your cum-theory:
take that same self-responsibility idea you have about a hiv-neg group & apply it to everyone: "I will forego cum in the ass (& use condoms for anal) because there is a high risk of aids or other disease. If I want to play with cum, I'll do it OUTSIDE of the ass, where there's no risk"

also there's the wildcard concept: why would someone with lots of safe opportunities go & do something risky & unsafe? = Hugh Grant is a well known actor that women find cute, who got busted with a prostitute in his car on a street in Hollywood. WHY would a guy with so much opportunity to get laid take such a risk?