For me, the contention is this part. Has the MC already stolen her?
In order to steal her, some base for laying claim on her must exist. A one-sided crush can work if I recall how NTR stories work. Does the MC have some actual expectation of being exclusive with her in the first place?
If you are visiting a prostitute, that prostitute working more afterward is usually not NTR.
If you participate in a gangbang/orgy/similar, the girls fucking others is not NTR.
Here, it’s basically the MC lusting after someone while she’s in a relationship, which is a bit of an unclear situation for me. On the one hand, he’s just a side-fuck, so there should be no claim and no NTR. On the other hand, John is upset when he sees the father with her, which speaks for NTR.
Personally, I don’t really care how it is labelled. Both tagging and not tagging the game NTR is misleading. NTR is misleading because it is just a single maybe-NTR scene in a large game. Not labelling is misleading because many people who avoid NTR also don’t like the NTR-ish elements of this game. It’s borderline.
I understand what your train of thought is, however in this particular case, the prostitution analogy doesn't exactly work, while the mom does kinda adopt that mentality and isn't much more than one besides the fact the plot stops her from going that next step.
As of that time, she had already engaged in a romantic relationship with MC it wasn't purely lustful.
Although as far as the term goes that doesn't really matter.
To counterpoint myself a bit, with the prostitution thing... that can actually be NTR, despite them being a prostitute because of how MC views it. (which is more purely in MCs view, unlike this where it is more than just MCs own view but an actual act, since the mom at that point was semi-estranged from the dad, since he had left at that time IIRC)
Like there are NTR stories where the chick is a prostitute and gets railed in front of the MC that fell for them. (is fkn retarded I know... like wtf is up with those MC beings doormats
).
What I will agree on is that it isn't Hardcore NTR, but it does fall within the realms of the terms meaning and definition. Since MC had not agreed to share them with the father, and how the story had gone implied he was out of the picture, hence his reaction.
And while i can't say this with certainty, I do believe she had promised or said to MC that she would be his or something similar.
As for whether or not this game should get the tag.. well it 100% should since the term is met. "LI involved with someone other than MC." (without his explicit permission, since that would be sharing). And yes that does apply to prostitute LIs... as dumb as it may be.