There's an NTR switch that you can freely turn on/off anytime you want in the game which will change the scenes so that the girls aren't having sex with other men, and will usually make it so that the MC is having sex with them instead. Like you said, Rita sleeps with other guys in the multiple movies she makes but the scene can change so that you are fucking her instead if you switch NTR off. As far as I remember, the only major girls that sleep with other guys if NTR is turned on are the Rita with strangers, like in the movies, your mom with your friend in the first part, your aunt with Steve, and Eva with Steve. The dev likes to make content for fans of both sides and some more girls seem like they may fuck other guys soon, so the list will probably grow in the future. There's no way to avoid NTR in the later parts if you switch it on, but turning it on and off whenever doesn't mess with the story, so its fun to check out both versions.
TLDR If you don't want scenes where the girls are sleeping with other guys then you are not going to see them, and will see a vanilla version of that scene instead.
