Do you have a large percentage of people who legitimately buy stuff from your website later claim they didn't? This is not your standard fraud model: with porn you see a remarkably large amount of "oh shit, someone (usually my significant other with whom I share my finances) just saw this charge on my card, which is totally not OK for whatever reason, so I guess my best strategy here is to totally disavow the charge and pretend it was credit card fraud".
A second issue porn has which most websites do not is related: customers actively do not want their statement to accurately state from whom they purchased the product, lest someone sees exactly what their fetish is simply from reading the credit card report. This drastically increases the chargeback rate from people who legitimately do not remember what they bought and do not recognize the charge (particularly if the service uses subscriptions).
If you want to accept payments for porn, you need a fundamentally different fraud model than for a standard website, no matter how weirdly digital or in high demand: this is not about scammers or about black card fraud, this is about basic lies and deceipt. I see no real reason to claim PayPal must go out of their way to support this.
A company which has is CCBill. They sign you up as a "high risk" Visa card merchant (which is an upstream notion supported by Visa), and then are much more hands on in the checkout process than a normal credit card processor, with a website people can use to look up and manage their subscriptions to websites (so like, imagine if all Stripe purchases required you to use their web form, showing up on the customer statement as only "Stripe", and if the customer could then go directly to stripe.com, enter their credit card details, see they had a subscription with you, and cancel it; to be explicit: PayPal does not have these restrictions, even though they might seem similar to you).
(To note: it does bother me somewhat that PayPal doesn't go out of their way to support some of this stuff, but as long as the actual credit card processing firms support it and there exist banks willing to allow it, I don't get pissed off about it as it is totally possible for you to do it yourself... it isn't like you are locked out of the actual network effect of credit cards, which is what is important; and given that there even exist easy well-supported full solutions out there like CCBill, it barely seems like a problem.)