It's kind of interesting that you're claiming a slippery slope argument, when the words we use to describe incest Oedipal and Electra complex come from wildly popular pre-CE fictional accounts of incest.

As for violence, that's because such acts on film were illegal. Pornography was tightly regulated before this era of free speech. Even now the UK, is constantly trying to maintain BDSM porn as criminal [1] and Australia has similar tight restrictions [2]. This is to say nothing of the countries where pornography is completely banned: China, North Korea, Indonesia, Pakistan, Bangladesh, Vietnam, Iran, Saudi Arabia, Syria, Turkmenistan, Belarus, Bulgaria, Djibouti, Egypt, Eritrea, Ethiopia, Iceland, Iraq, Kyrgyzstan, Malaysia, Malta, Myanmar, Sudan, Thailand, Turkey, Uganda, United Arab Emirates, Uzbekistan, Yemen.

So if the question is why do you see more of Y in XXX, it's because it is now allowed so content is created to satisfy needs that were already there.

[1] https://reason.com/2014/12/02/uk-bans-fetish-porn/ [2] https://www.kptlegal.com.au/resources/knowledge/pornography-...