As an unfortunate Netori enjoyer I can say for myself that i mainly enjoy the corruption of the "pure love" between the characters. Where the characters, who usually seem pure or strong-willed in the canon stories, slowly become corrupted by things like carnival pleasure and enjoyments.
I have read and seen countless manga/animes/lightnovels, where the mc and fmc are pretty much already chosen since the start and the other side chars are just for some rivalry/character building/storyplotcontinuations.
so most of the time its just pure vanilla romance mixed with drama.
So ntr stuff is like a fresh of breath air, seeing how those female characters would act in a less vanilla/wholesome environment, but in a more corrupt and darker one. where problems are not as easily solved as in stories.
for sure ntr plots often make the characters more stupid and ppl tend to say "its always the same", but ppl seem to forget that most problems in life are usually like this: status, money, relationships. and ppl who are at their weakest or their breaking points are usually the easiest to manipulate and blackmail.
Sure i dont follow the SAO stories, but im pretty sure, most of the characters there are still young, not even 18?
Just because they went through some life and death situation in a game, doesnt mean they are suddenly the most mature ppl out there and immune to any kind of shit they are thrown at.
Asuna for example is still a girl, living with a strict, overprotective and overbearing mother, with no father figure, coming from a "prestige" household and a lot of social pressure, trying to maintain a healthy relationship with kirito + trying to help and stay with friends with inoda + paying back her debts for him protecting her from thugs. also her having no real friends to whom she could share her problems with, kinda gives her some tunnel vision.
Kirito is still just some guy who played a lot of computer games, doesnt seem like overally academic and also seems to live in a small apartmant, with no job income, or any achievments except for having survived in the sao world.
asuna being from a good household and her mother not liking him for various reasons, obviously will eat at his self confidence further.
yes, im aware that im overthinking this kind of stuff anyways, but i have to say fujino is actually portraying the ppl quite well here in a more mean world, where social problems and corruption are way more present and powerful. especially for the character ages and in japan...