I'm not convinced of the causality here. It's more likely that societies that are so repressed as to ban pornography also are too repressed to handle sex-related issues, whatever they are. Rapists are a small percentage of the population, most of them fall under cluster B personality disorder, and their prevalence does not change much across cultures.

Openness about sex allows two things: first, abusers can actually be caught, and stopped from re-offending; second, the unacceptable nature of sex offenses is on public display.

When all sex is censored / hidden from view, there is no social distinction between normal and criminal behavior. Consider, when having sex with your fiancée before marriage is supposed to send you to hell, what's worse about diddling the little cousin? Of course, not to normal humans, but that's the kind of things that goes into cluster B's heads.