Part of the issue as I see it is a concept that I've harped on before, which is an excessive mentality that the Truth as one human sees it, is The Objective Truth, and only "those wrong, sick, or messed up, immoral people" live any other way.
I have never understood why so many cling to this, it seems like it would be a massive headache to constantly have to judge everybody who shows even the slightest diversity of thought or aberration of behavior, but apparently that's what folks do.
My ex was one who was obsessed with the notion that his partners would always cheat eventually, somehow, and was on guard, paranoid, on the lookout for it. He will say that he is THE paragon of virtue when it comes to loyalty, honor, and integrity, and no one can match him in this, ESPECIALLY not a woman. So years and years ago, I had an interaction with a young man who was a bajillion miles away and across an ocean, online, where I was mostly exercising my brain in writing smut for him to enjoy, while trying to also boost his confidence in himself so he'd go out and make more music and make more human connections. That was classed as an emotional affair and caused a massive explosion where he tried to put me out of the house, and required a huge display of contrition and for me to take his (rather abusive) "punishment" and not be trusted for a period of time, for him to come to grips with the whole thing. I had to heavily explain, justify and disclaimer every conversation or interaction with any other male in the world for him to be accepting that I was not about to stray.
But I was "allowed" to have as much sex as I wanted with another woman if I so chose, because women "didn't count." So he said.
And this was "duh of course" to him. It was just something that, had I the proper amount of moral fiber, I would just know, without needing it explained to me. That was how "fidelity" was defined to him. He also believes that jealousy is a natural, proper part of any relationship. He wishes that he had a partner who would get jealous of other women coming on to him. The fact that I had told him numerous times that I didn't really care if he had sex with other women, so long as he kept his major time and money investments with the family (because, children)...to him, that was proof that I didn't love him. Had I loved him, I would have been ready to fight any other woman who so much as farted in his general direction. Or whatever.
He also says that men and women cannot be platonic friends, unless the man can classify the woman as a "little sister" or unless she is ugly. What she thinks or feels in the interaction is not considered. It doesn't matter. The man's thought processes define the interaction. Because of course.
He is quick to defend and justify when men cheat, and quick to vilify women who do. He sees men as people, and women as strange and frightening "other" creatures. Not really people. I'm still recovering from the psychological harm from living with this mentality and trying to remind myself that most men don't necessarily think as he does. Although he was always quick to tell me, when we disagreed, that he talked to his male buddies and they all agree that his way is how it is.
But he is a glaring example to me of someone who brings a set of strongly held beliefs to a relationship, and simply tries to shoehorn a partner into a mold of his making. And I see lots of people, even if their mentalities aren't necessarily quite so toxic, doing more or less the same. I see this form of "fidelity" as nearly a kind of insanity...and I want nothing to do with it.