I feel like you're basically just describing the idea of corruption / 'slut-ification' -- the girl ending up into more things than she had been before, at least specifically with the guy stealing her if not in general -- just dialed up to the extreme. I certainly enjoy seeing that change being worked, but I don't feel like it being dialed up to be more and more extreme does much for me past a point.So, it doesn't correlate how much nasty things the antagonist subject the girl to?
I mean, its at least understandable that girl might switch MC to a better lover, but if she switches to a guy who take a morning piss down her gullet, isn't it multiplying the feeling of being NTRed?
So its like, for MC she was just a vanilla hand-holding GF but for a antagonist she is submissive cunt dripping masochist?
Like, I feel like that's a big part of why these sort of stories tend to have the FMC be almost ridiculously 'pure' to start with, so that even with an end-point of a (relatively) tame "has a really big sex drive, constantly wants to be doing it with the bull", there's still a very stark contrast to where she started.
[...]there's no feeling of loss if she was fine with any of it and she wasn't who you thought she was.
I agree with a lot of what you seem to be saying, but think I might have a slightly different take here. Strictly speaking, the way you phrase it, I do agree; if the girl was just hiding her 'true' nature all along, and jumped at the chance to act on it, then that's not NTR. It's like I said in some earlier post as to why I consider 'cheating' a different genre.If she ends up being a completely different person, why would he care?
To me, though, what you get in a lot of NTR that hits this sort of thing isn't the reveal that the girl wasn't actually who she pretended to be; it's that who she is is *changed*, remolded, generally very much against her will (at least at the start). Taken to extremes, it basically ends up being a sort of 'death of self': despite the girl still being alive and well, the *person* she was can be irrevocably gone.
(As a side note, you see this sort of dynamic a ton in stuff that's more overtly within the 'brainwashing' type of genre, which honestly NTR shares more with in terms of vibes than I feel like some people would want to admit. From a rational / emotional point of view, it's incredibly fucked up and disturbing, but if it's done well, it can be intensely arousing at the same time; again, contrast!)