I think this is a rule to keep payment processors happy that won't be enforced. How are they going to find these sexually explicit videos among all the nude videos they still allow? Maybe they have software that can distinguish between a nude video and a sexually explicit nude video. If not they need to either review videos that are posted or rely on users to report sexually explicit videos. I doubt they want to manually review all uploaded videos, so that leaves user reports. Who is going to report an onlyfans account they subscribe to for posting an explicit video? Probably nobody.