The most effective regulation here is from payment handlers.

Not sure what eventually became of this, but MasterCard shitcanned PornHub a few years ago. See https://www.bbc.com/news/world-52543508

I also see a statement from 2022 reinforcing that decision. https://www.mastercard.com/news/press/2022/august/mastercard-statement-reinforcing-adult-content-standards/

I guess even that's not very effective since PornHub is obviously still in business. I wonder what their numbers looked like before and after.