So in your example, the NTR comes from the fear/anxiety around having a woman stolen by sexual means from the protag, rather than just a woman being sexually conquered or corrupted. It starts out forced, but eventually she likes it and then prefers the situation to the one she was in before with the protag. NTR is the anxiety (and confusing arousal!) around the possibility of loss of sexual partners to other men.
I don't want to hijack this thread more than I have but it's actually an interesting intellectual rabbit hole to go down regarding something called Sperm Competition and the origin of why this is even a fetish for men.
To keep it simple: it is evolutionarily advantageous if a man upon suspecting that 'his' woman has had sex with a rival male to react by actually wanting to breed with her more and immediately, rather than to feel disgust/anger and turn away from her. That way his 'sperm' can compete with any rivals that may be inside her...there is more to it, but it gets graphic and involves penis/teste size etc.
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Kurotori