In the end, the genre "NTR" isn't based on "what men or what women does" but "what the Protagonist does and what happens to him". The main protagonist of your story - for most webnovels often the self-insert character - is the focus point for your genre tags. Whether a different view point or argument can be made for harem-situations or other stuff doesn't really matter in this case. It's a simple question of what happens to the MC.
That's the reason why a harem - even if netori happens - isn't marked as NTR. Because the genre tag NTR is based around the fact that "the protagonist loses his girl". It's a warning to readers who want to self-insert into the characters: "Your heart will be hurt."
NTR as genre marks a loss the viewpoint-character has to bear with. Whether that is through passive actions, active actions, threats, etc doesn't really matter for the genre tag. It's about whether the story "is a happy one" for the MC. In fact, there are multiple harems where the MC will steal women from other men. But those aren't NTR under the tags because that's "a good thing happening to the MC".
It's the same way you won't put "overpowered MC" as your genre/tag when it's a side-character that is powerful...
Look... Harems have female NTR in that, but since most of the people reading Harems are men, they don't care what the female feels so to them it's not NTR and as such won't be defined as such. The end. Done.
The overwhelming majority of harem don't have "female NTR" in there. Because usually the MC picks up virgins who fall in love with him left and right. It gets a bit muddy with smut-harems where the tropes/themes change up a bit and characters also switch, but harem aren't really NTR. Especially as the overwhelming majority won't even have sexual relationships before the harem is established.
It would be some kind of cheating/NTR if the MC gets together with the first girl and afterwards adds more girls. But harem MCs are usually either rather open about their desires ("I won't be only yours") or you have multiple girls near the MC before they take the next step (and then there might be more people added afterwards)...