Except NTR doesn't mean cheating or cuckolding. 寝取られ or netorare is made from the characters for sleeping (ne/寝) and taking (toru/取ら). That's what netoru or netorare means, stealing through sex. So if you have a mother who sleeps with her child's bully and abandons her relationship with her son that's netorare. If you have the MC's big sister who he's always been extremely close with and she ends up being seduced by a villain who steals her away from the MC and essentially destroys their close sibling relationship then that's netorare. If the MC has a close childhood friend who ends up being stolen away and seduced or corrupted by a villain to the point that her relationship with the MC gets destroyed then that's also netorare.
What makes something netorare is there needs to be a victim, a villain and a heroine. There needs to be stealing, either through seduction, blackmail, rape or corruption. Lastly there needs to be the destruction or significant subversion of a pre existing relationship.
Some guy marrying the MC's mom, respecting her relationship with her son and not trying to subvert it or steal her away from him is not netorare because there's no villain or victim and the existing relationship is preserved.
The mom trying to confront her child's bully, eventually being seduced and corrupted until she starts joining her lover in bullying her child is netorare. There's villainy, there's a victim, and there's stealing leading to the breakdown of a pre-existing relationship.
Best explanation I've ever seen about NTR is it's not the fetishization of cuckolding but the fetishization of loss.
