I agree in one sense both sides of the coin, but many players do not like seeing their LIs touched by anyone else, its ok for MC to do it hence why a lot play it to have lots of options but the LIs cannot, selfish yes, but thats the way many like it.
I understand people who can't stand sharing LIs in any way (or having them stolen), takes being a bit of a deviant to overcome both biological and social conditioning. Power trips are easy dopamine. But it's interesting, as you say, that even harem games attract people for the same reason sharing, incest or gender play can - its transgressive nature.
The hard thing about being drawn to transgression is that, once you cross a line, you need to go further down the rabbit hole, because what you already experienced doesn't cut it anymore. It's like addiction on steroids. Easily becomes the two largest crowds here at f95, those who roar against and rationalize it to dispel the allure of taboo, and those to whom no amount of outrage and scandal is enough.
I also understand that many do get off on sharing and being cucked
My issue here is that I see sharing and being cucked as two different things. They may coincide, at times, but while I enjoy certain sharing scenarios, I hate the cuckold fetish and also have very little interest in "sharing the girl" as power play. What I enjoy is when sharing is part of a couple's dynamic, something that brings them closer together, not apart - as long as there's no MC jerking off to his girl being fucked by others. Since it has to be sourced from within the couple, and have the sole purpose of satisfying the couple - not either one's selfish cravings - I don't count open relationships as sharing, and other guys must be objectified and have limited agency. I hate to see a rival score. NTR is not my thing.
In general, I prefer a story that isn't about sharing and where that may come as a result of the MC and FMC both enjoying mutual teasing. When the game makes them crosses the line into either of them seeking others for their own satisfaction, it's no longer about the couple and - however legitimately tied to the story it may be - this doesn't interest me any longer and may take away my interest in the game as a whole. Doesn't even have to involve MC humiliation and bullying.