CMV: the conception of “cheating” (infidelity) as a top tier relationship-shattering offense is an arbitrary social construct, and attempts to explain what is so intrinsically bad about it are typically either circular or fall to the most basic analysis. We could just as well collectively agree to think of it as not a big deal, or as a bad habit but not an unforgivable one, or in any number of other ways, and our thinking would make it so.
Example of an argument that fails basic scrutiny: “Cheating is a betrayal because it is dishonest: you are agreeing to follow a limit your partner has set, presenting yourself as following it, and meanwhile doing nothing of the sort, so the relationship is a sham”
This sounds plausible, because yes dishonesty is itself bad in a relationship. But it says nothing about why that -particular- limit would ever be set in the first place.
Also I suspect that the dishonesty is rarely the true objection, just the easiest one to defend intellectually. If someone just openly said to their until-now monogamous partner “I’m going to go bang some other people now, if you don't like it you can dump me,” most people would still call that cheating and would definitely not think it was okay just because there was no dishonesty.
Ultimately, there are two social messages involved here, both more or less explicit : first, that people are best off in pairs with a high level of intimacy and sharing, rather than on their own or in larger clumps ; and second, that the definite or distinctive feature of such a pair is the exclusivity of sexual activity.
Now, you may call that an "arbitrary social construct", and perhaps it is, but people live and die by such things.
Now, it's obvious that humans are social creatures who cannot thrive alone. But I suspect the emphasis on pairs has to do with the ever-growing difficulty of achieving trust and intimacy among multiple people.
I've seen it suggested that the ratio is at least geometrical ― if the difficulty with one other person is unity, then with two it is four, with three it is nine, and so on. Honestly I think that's reductive. It depends very much on the individuals involved.
Now, many people attempt to connect the emphasis on sexual exclusivity with land inheritance in agricultural societies, but if you look at the incredible complexity of marriage customs/laws among hunter-gatherers such as the Australians, that idea just doesn't hold water.
There seems to be good reason to believe that early humans lived in bands or small groups, with sexual relations being fairly free and open. How we got to this point is largely a matter of speculation.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko