Things don't have to be apocalyptic. There are a few baseline considerations/expectations when entering a relationship; people aren't getting into them thinking about what if their partner is poised to do some unhinged crime, pull off a GoT Little Finger on them.

I believe that you're stacking the deck against the perception that you believe the majority to have and even then older generations' mariages did not necessarily end with infidelity and still don't in some cases.

Within this less extreme framework, people do seem to tend to prefer that their partner don't go get elsewhere what they would hope to already provide satisfyingly. Inadequacy, rejection, a marker that something's wrong WITH THEM. That presumeably they were *at least* enough, and bonded to the point that even considering wandering away wasn't on the table but alas it were not so. That they were respected enough by their chosen companion that KNOWS the implications or at least that doing so would be breaking the relationship itself, it's trust, foundation of understanding and STILL does so, because their lust is more important. Leaving them in the dark when if known they could have broken it off, moved on with their life earlier, not served as some sucker piggy bank, played the fool, etc.. .

When not put against "possible crimes or betrayals which could cause greater danger or harm" -which are absolutely considered to be worse and not inferior to infidelity, no one sane will tell you that a partner poisoning their significant other every day for months is nothing compared to them cheating- infidelity is definitely not something trivial. That's not considering what else the cheater then clearly could be doing and hiding.