Don't want your girl to get involved with other guys? Then, do the girl's route and save her.
The game will not have NTR, the developer has already made it clear, NTR means betrayal, so if you do the FMC route correctly, nothing will happen.
Now, about the bad endings, how can the FMC commit cheating (NTR) if you have no connection with her? Her having relationships with others in the bad endings does not mean it is NTR. Since you ignored her route, you have no connection with her.

