Okay I'll make it more specific, I will revise my question, since this game is female protagonist does the kind of ntr in this game is cheating ? because Female MC has a boyfriend right ?
It's still not an easy question to answer, because the situation is unusual. If you want a simple yes or no answer, then yes . . . technically, it is "cheating" but it isn't presented as such, because it didn't quite
happen that way. It's been stated by the dev that events in the story are taken from the lives of herself and her real life friends. It would almost take as long to explain as it would to simply play through the scenes, and experience it firsthand. If typical NTR content is a prerequisite for you, you might want to 'pass' on this project. The NTR content is very minor, as compared to typical fictional NTR content in games, and I'm not aware of any future NTR content. There really is no simple answer that I can find. (Actually, there IS a bit more NTR coming up, but again . . . it's very unconventional, and difficult to explain without completely spoiling the story).