The problem is you can't prove your age with BTC.
Teenagers don't prove their age when they use their Dad's credit card to sign up for porn either, get what I mean?
A porn site will ask a user to verify that they are over 18, if they do so then it's on them whatever payment method they're using.
At least with bitcoin your wife can't check your statements
i'll be dead when my dad found out there are some statemant for paying porn.
anyway the majority would be thinking twice before purchase some porn in internet, due to the privacy is the most important that's why they used bitcoin, but this is not a big adoption only small % of the porn site in this world that accept bitcoin as payment method.