Hentai genre netorare (NTR), in mainstream Western terminology, is cuckoldry. Netorare occurs when an outside party takes away a lover from the protagonist and makes that person his or her own.
As I’ve stated before, Japanese non-live action media provides escapist fantasies for many of its consumers. With hentai, that includes a variety of fetishes, such as netorare.
Aside from it arguably being a fetish–and fetishes can be exclusionary because of individual tastes, I believe there is a reason to support why there are many people who hate this genre: human instinct. Such people find NTR unappealing because such behavior is connected to real-life dominance in men and hypergamy in women. Hypergamy means women instinctually (read as “mostly subconsciously”), especially once they feel they are ready to have serious relationships and to have children more importantly, seek out men who have the resources which will allow them to support themselves and their offspring. The male equivalent dominance is connected to this from a behavioral standpoint which helps to support the general consensus that younger women like bad guys, even those who do not yet have resources. Hypergamy is a behavioral instinct and, as such, it is subject to behavioral cues. Of course, animals also have these instincts. Males of many sexually dymorphic species must impress potential female partners before the females offer them the chance to mate.
Some male and female anime et. al fans partly turned to these media because of their dislike of certain aspects of life, including romantic interactions. Some would argue, for example, that feminist laws have harmed the dynamics between men and women, and long-term relationships are no longer viable. Additionally, I see a time spectrum where each such person turns to anime and so on for a pure escapist fantasy for varying periods. Their escapism can be temporary; their escapism can be permanent. Even with the ease of power-tripping with hentai because of the variety of themes available to viewers, there are many viewers who want to feel good about power-tripping, and so they only watch vanilla hentai. Otherwise, it reminds them of what they disliked about the real world. They seek out character dynamics in hentai which reinforce their idealizations of what romantic relationships free of feminism and other indoctrination could be: typically honest, good-natured, and straightforward without some sort of not-so-hidden motives from either party. This, we must admit, is what netorare is not.