Mmm I see, let's go by parts:
1.- Lesbian is not ntr, whoever likes it likes it.
2.- The game does not have the tag "romance", I assume that the relationship of the MC with the girls is more physical than emotional, I read something in a post that said that this character said something about an open relationship or something like that from the beginning? bothered).
3.- Even after that scene I doubt that that npc is going to "take" his girls from the MC leaving it more like a casual encounter of a girl with no real commitment to the MC (not his girlfriend, family or REAL love interest ).
4.- The game doesn't say harem so the MC can have sex with all of them but in the end they are not actually HIS girls.
If what I mentioned above is real, I don't think it's ntr (just like it isn't in Superpowered or Oppai odyssey).
Conclusion: In general terms, it's not ntr, and as long as the MC doesn't end up like a cuckold without touching a tit or humiliated and enjoying that, it's fine with me.
Good luck.