Ok, I feel is a good time as any to devise some sort of, like, treaty over this debacle.
Everyone's been having their contributions to this discussion so I guess I can also throw in my dogs in the race.
First of, NTR is a very specific type of cheating fetish. It's requires a series of specific factors to count as such, and needs to be taken into account as its own beast.
These aspects are:
There needs to be an established familial, romantic or platonic relationship, featuring two characters who have feelings for one another.
One of the two characters then needs to be "taken away" from the other by a third party, usually against both of their wills. This can involve mind control, blackmailing, outright sexual violence, that sort of shit, but it's never, per se, voluntary on the "taken" part.
The person who has been "stolen" then is permanently "ruined" by this experience, usually leading to all sorts of nasty repercussions. Pregnancy, Prostitution, Mind Break, Suicide, that sort of shit.
And last, the most important part, the whole ordeal has to be depicted ALWAYS from the POV of the person who's having their SO "stolen" from them. The whole point of the genre ISN'T, as someone might assume, watching some random people fucking, that's now how this fetish works. The whole focus character is the loser who gets their lover stolen from them because they weren't good enough to protect them, it's their despair or impotence that fuels the fetish, either with you "identifying" as the one who has their lover stolen, or the one who is done the stealing.
You'll notice how the "stolen" has no, like, relevance in this equation, and that's because they aren't an actual character, they are a prop to perpetuate some weird psychosexual bullshit between the NTRer and the NTRed.
There is a reason if Griffith only looks at Guts as he's raping Casca.
Since, again, the feelings of the one who is getting their lover stolen are what ultimately matter here, the one doing the stealing is usually the most disgusting piece of shit ever created. If the one doing the stealing was attractive or fit, then the reader would feel less "betrayed" by the situation. It'd be seen as an inevitability, and it would grant the "stolen" a certain degree of free will. For example, Ophelia and Derik fucking ISN'T NTR, because everything is happening of her own free will and she is consenting to it without coercion, she is an ACTIVE participant in the scenario, even if the MC is getting "Cucked."
This is awkwardly enough more common in Japanese Porn rather than Western Porn. Most western porn with cheating content tends to play up the female character "cheating" on their partner, especially if in some kind of "bisexual girlfriend" setting, any temptation being usually on the purely physical variety. Even the more "extreme" types of this content, such as, random example, the old Pornstar Punished Brazzers series, where there is a major degree of coercion and violence, don't tend to deploy truly hideous individuals either inside or outside (That fucker James Deen notwithstanding, that fucker).
Japanese Animated Porn, on the other hand, can straight up make up some of the most hideous motherfuckers ever devised, a concept that is seemingly replicated here too with Hunter and to a lesser extent Kevin, whose physical hideousness is but a metaphor for the fundamental core theme of this game, which is "self improvement" (Sterling actively decides to become a better person, Hunter decides to coast by and live off his cock and "shortcuts" to get women). This, again, is done to generate a far stronger reaction from the public who, again, is probably self identifying on the main character in a way.
This to say, the appeal of NTR, which I personally never really saw really, I'm more into women being sluts by their own free will, much sexier if you ask me when a female character has actual fucking agency, is, again, all concentrated on the way the situation will make the MC feel, and therefore the way the reader can jerk off to it, nothing more, nothing less.
