- Edited
SlaveOfTheLord I meant to say that it's more universal (?) than cultural. It doesn't happen in specific cultures but also all over the world. In fact, I think it might be imported from the West, for example in the Meiji era in the 19th century where Christian values were being embraced. Christianity, in particular, embraced the whole chastity thing (basically you have to marry the girl you sleep with if you took her virginity in Christian tradition, or her father will shoot you with a shotgun), so the whole purity thing is more of a modern thing in Japan and China, and not some ancient tradition stretching back centuries.
Also, English speaking Western readers appear to subscribe to this as well - I see comments screaming "NTR!" the moment the heroine or one of the harem members sleep with some other guy. Hypocritically enough, it's all right for the male protagonist to sleep with multiple women, but readers begin raging about NTR the moment a harem member sleeps with another guy. So it's less culture but more of a niche section of the audience all over the world (do we call them incels? I'm not sure), especially those most likely virgins who don't really go out and interact with real women and have this idealistic notion of purity.
Most normal Chinese and Japanese, just like most normal Americans or Westerners, don't really care about purity or chastity. Just those "incels" or whatever you call them.