I think it's just one of those "grass is greener" types of situations. Most guys wish they had a bigger dick, but guys with bigger dicks find that most women don't actually enjoy getting punched in the cervix. A dilemma, for sure.
I don't think it's NTR-style appeal though, just group sex really. The NTR appeal is seeing the main character's girl taken away from him by some other guy. She's obviously not being taken away from him. I think it's closer to swinging, maybe, only I'd hesitate to call it that as she was sorta forced into it.
I mean, NTR is specifically having the girl "stolen" away (tori from toru, to take/steal). He didn't have his girl stolen, and it's all under his control. And if this was exhentai, the netorare tag would get downvoted into oblivion.