It happens in Japanese porn as well, which leads me to think it's a cultural thing.
It's very annoying. When they say 'stop' and the character keeps going, I can't help but think "BUT THEY JUST SAID STOP, SO JUST STOP!!"
It would be funny to see their reactions if the act did stop. If it was me in that situation, I'd high-key ask them every time "so, are you still okay with this or should I leave...?"