its not just the sleeping with someone else ppl hate ntr for its cause in most cases in doujin it is the girl being tricked ito it in mostley a dumb way i.e. guy made a mistake so the the wife is blackmailed into it and the sex is usually rough and intense and breaks the girls mind down till they only want that pleasure even recording it when the girl is broken enough and sending it to the guy instead of idk goin to the cops or talkin to the guy that shes getting harrased....not gonna lie ntr sucks but with they way they push it and make the art for it better than other stories in doujin i think its just too popular in japan