In my opinion, NTR, sharing and cheating are three different things. Traditional NTR, like in japanese games, is a lot more than just cheating or sharing. The game make you invest a lot emotionally into a character then make you watch this character being fuck by some other people. It's a kind of emotion manipulative game for people who like the horns they have on the head and for those who like to be blueballed. NTR provoke strange reactions in me, on one hand, it may excite me a lot if done right, on second hand, it make me feel culpability (which is strange as i often play game with darker theme and don't really feel that) and on the third hand i just don't really like them anyway (because i'm a woman, story is important for me and NTR is often use as an easy plot by lazy dev with no creativity).
Even in real life, it's complex. There is an increasing number of people who build relationship outside of mandatory cishet monogamy. It can be without every people consenting (cheating) or with every people consenting (swinging, open relationship or polyamory).