While I don't want to defend the crazy guy here (I love Eileen and think his stance on her is ridiculous), this is a bit of a strawman. No one's said that people who are not LIs getting laid is a problem (except the crazy guy, and I don't care about his opinion).
What people don't want is LIs (or future/potential LIs) who
do talk explicitly and in detail about past sexual activity. I forget which game, but there was one where an LI went into graphic sexual detail about past relationships while talking to the MC, which was, shall we say, not appreciated by most players.
For example, my anti-NTR opinions are:
- I don't want to see an LI or future/potential LI talking about how big her past lovers' cocks were (or anything similar)
- I don't want to see an LI or future/potential LI talking about how much she likes being fucked by other guys (or anything similar)
- I don't have any interest in seeing scenes of people having sex unless everyone involved is either the MC or an LI/future LI (not potential; they must be a definite LI or an optional LI) of the MC (there are some anti-NTR fans who don't like it when multiple LIs have sex without the MC; I am not one of them, but they definitely do exist as a significant but not universal portion of pro-harem/anti-NTR people)
Guys other than the MC could be having sex 24 hours a day, but I (and most anti-NTR people) wouldn't care as long as we don't have to see it or be told about it, especially by an LI or future/potential LI. This is why it's so important to clearly delineate between future/potential LIs and women who will never have any interest in the MC.
To expand on that last part: one reason devs generally make non-LI women unattractive is because if they make an attractive woman,
someone will ask them to make the woman an LI if she isn't. That can lead to negative situations. For instance, there is a game where one of the first characters the MC interacts with is a pretty woman who is in a relationship already with a guy who the MC is friendly with... But she also
immediately flirts with the MC and hints that she's attracted to him. One of the next scenes (and the
first lewd scene in the game) is a voyeurism scene where the MC watches the girl and her boyfriend fucking in an alley. The dev expected this to be appreciated 'because it's a sex scene and she's pretty', but there was a lot of pushback that resulted in the dev adding a skip option for that scene, and it's turned a lot of people off of the game anyway, because teasing an LI and then having another guy immediately fuck her as the first lewd scene is really shitty to do for non-NTR games.
Anti-NTR people generally hate multiple protagonist games categorically, almost as much as they hate NTR games. So it's not usually particularly relevant. But the answer is "if more than one protagonist is interested in an LI (and neither the LI nor the game make it clear that the LI is 100% off-limits to all but one of those protagonists), then it may not fit the definition, but anti-NTR people will hate it just as much as if it was straight-up full-on cheating/cucking NTR."
