It's NTR. The cucked doesn't always have to be aware that they're getting cucked. This website laid on the definition. And people still go around trying to argue otherwise. Why? Do they feel offended when NTR is labeled on things they like?
Also Mitsuyo is a bitch. I really hate these types of characters but they have to be this way for a generic NTR story. Would be interesting if she took this experience and tried to teach it to her husband, but nope!