Like I said, there is a lot of sub content of NTR. You are talking about Netorase.
Netori: What is Inoda doing is Netori. "Sleeping with a girl who have boyfriend." It is NTR also. But that is not cheating because Kirito and Asuna both want this experience.
Netorare: For example, "Asuna sleeping with Inoda because Kirito wants to watch this and jerk off." This is also NTR.
Netorase: "Asuna sleeping with Inoda. When Kirito saw this, he feels disgust and wants to kill himself but Asuna keep doing it because she feels good. That is also NTR.
There are other NTR types. That's why I told you, please use Google.