I used to blame it on the globalization of porn with the internet. When porn was directed to sell to American men in video shops it was all maids, secretaries, and cops -- basically power fantasies of one sort or another. As the wide world started consooming (cough Pakis) it became all fuck-your-sister, fuck-your-aunt porn.

That may have been a bad take, though. it is possible there was always a demand for this stuff but the taboo and questionable legality kept the incest down. My understanding was that it was questionable whether incest was legal to show on film, even simulated. Apparently they're not worried now. Still, you can put a girl in pigtails, but you can't say she's 12. It was also verboten to simulate rape. It could just be the walls of decency breaking down.