Hypothetically I remember a set of three connected "works" where a guy's mother was NTRed to the point that she willingly became pregnant and just kicked him out, so he went out and NTRed a guy's GF, and that guy NTRed someone's wife. The mom had some regret after seeing a Christmas photo of her and her son while nursing the baby.You can't NTR the NTR-man, that's crossing the line!
Or something like that. I may have "heard" about it wrong.