I don't get it

I really don't get why in almost every yaoi they feel obligated to make the bottom say "stop" or "no" all the time when they are fuc*ing. Why would it be fun to have s*x with somone who seems to not enjoy it when you're not a rap*st ? I mean, it's not sexy at all, I can't wrap my head around it. And it's not like I can avoid those yaois, it's almost easier to win the lottery. It's like it's a rule the writer has to follow. I know it's not a big deal cause it's not in real life and I can just change the dialogues in my head or skip those parts but really, I don't get it, I don't get where it's coming from