Well first is that the NTR tag here isn't that usual description people might think of when they think of NTR.
"Ntr [Designed to cause jealousy by having the romantic interest involved with someone other than the MC.]"
Basically, because whether the player deems it to be something meant to cause jealousy or whatever is a variable that can't be strictly determined, you're left with: "The romantic interest is involved with someone other than the MC". Not only is that a possibility in this game, it's forced through unavoidable rape scenes. Not warning people of those things is being disingenuous. The easiest way to explain that would be to add an NTR tag/warn people about that. The kind of people that would be turned off by that would likely be the kind of people turned off by even the usual definition of NTR, correct?
You're free to disagree, just spreading the thought process for those curious. You could avoid wasting a lot of people's time by adding the tag. I can't think of a better way to avoid that beyond warning people like this or adding the tag. My goal is to reduce the amount of people who would DL this and find out it has shit like that and regret it. Until there's a:
tag, that's all I've got, lol. Not harping on or attacking anybody into NTR, it's just the best fit here to warn people. If anybody can come up with a better solution, by all means, go with that. Right now I don't think the current tags adequately warn people about the content of the game.
TLDR: At the end of the day, the girls the game focuses on get banged by other dudes and it's completely unavoidable. Closest description to that to warn people would be NTR, even though it might not fit people's exact description of NTR.