I can't believe I'm actually writing this. Because of the ongoing debate of what NTR genre is I've decided to describe my point of view.
NTR comes from japanese word 寝取られ (netorare) with 2 subgenres: 寝取り (netori) and 寝取らせ (netorase).
2 kanjis are presented here:
1) 寝る -> "to sleep" which has (as in english) literal and figurative meanings. Here is used figurative meaning: to have sex.
2) 取る -> "to take" which, if we look at word netorare, is written in passive voice 取られる (torareru) -> "is taken".
Thus, we have a compound verb with meaning close to "(someone) is taken (from somebody) through sex".
This meaning defines means and ends of the genre. While it's all clear with "means" part where change of heart should happen through pressure of carnal desires, the "goal" part where woman has to be
taken from MC is debatable. Imho, female character is taken when her original relationship connetion with MC has been diminished or destroyed all together while new connetion with the person who "took" her has been established. And relationship with MC shouldn't necessarily be as lovers, family bonds seems legit target to me as well.
A few word about subgenres.
Usually nerori is seen as "good MC take poor woman from bad husbund/lover" and I don't think it's correct. While netori no longer has passive voice in「取」part, it still has the 「寝」part which should be the main reason (means), otherwise it's no longer a netori game/VN but... some sort of melodrama I suppose. Good example of genre would be studio pork, especially their latest VN
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.
Netorase has causative verb form (to allow/let/make) which means "(someone) allows/lets (somybody) to be taken". I guess this is as close to cuckold genre as you can get. Good example here would be works of Atelier Sakura, specifically
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series.
I hope this textwall will help somebody to understand the genre better.