There are two levels of questionable: what you mentioned works for things like less common proclivities but not for things like revenge porn or other cases where it’s not just a question of what a user is willing to see.

because asking the average user to fine tune their experience to avoid questionable sexual content is going to piss a ton of users off who expect this to be the default experience on the site. So Facebook does a calculation and the result is that removing that stuff is easier than having a million people complain about being exposed to it and that's that.

The same reason any other forum or platform moderates their content and usually removes sexual content, you want the default experience to be good for the average user on the site.