Man, what is it with Japanese Mangakas punishing their male leads, but never punishing their female leads.
NTR is a genre that focuses on a Japanese male audience. It seeks to maximise emotional suffering by empathising with a passive protagonist, increasing his sense of helplessness in the face of what is happening to him as he is humiliated. It also explores the fear of not being good enough, of being replaced. The Japanese female audience is not interested in reading these stories, where FMC is only a sexual object. How many people would read this if you removed the sex scenes?
It is a genre that sells, as we mentioned earlier with Katsura and ‘Entanglement: My Truth and Your Lies’, which are two very similar works. In both manga, the MC cannot have the FMC. In ‘Entanglement’, he is humiliated in different ways; she has sex with others, even with the protagonist's best friend, and when the protagonist has the chance, it is simply impossible for him to satisfy her, he can only watch passively. His conclusion was, ‘I know I'm alive because I suffer.’ This is also seen in hentai. I cannot get my wife pregnant, so someone else comes along to get her pregnant. Here, the humiliation is the replacement of the husband by the lover, as in this manga with the partner swap.
From our Western point of view, we expect progress, consequences for actions, and for the protagonist to be an active subject in his own story, and when this does not happen, it creates frustration.