Netorare (寝取られ), often abbreviated as NTR, is a controversial subgenre in Japanese adult media that focuses on infidelity and the emotional devastation it causes. The term translates to "cuckold" or "taken away by sleeping," and it describes a story where someone's romantic partner is seduced or taken away by another person. The genre is designed to evoke complex and often painful emotions—like jealousy, betrayal, and helplessness—which some players find thrilling in a safe, fictional space.
Here's a lengthy article on the topic: