I understand that people don't need to be in a relationship for it to be NTR, but this most definitely isn't NTR by any means. Gen clearly doesn't give a shit, it's one sided by HER part, and the guy is clearly pushing them together anyways.
Not to mention, it's abundantly clear that the author won't do any such thing. I'm guessing people are only pretending to be worried. But I might be wrong since manga/anime comment sections tend to have the dumbest people in them