To Stingers point. Mankind has evolved or adapted to compartmentalize EXTREMELY WELL.
Likely evolved, IMHO. We need to be able to do this, and do it well, to function in the society that we have today, but, more importantly, in history/pre-history. Imagine if you were unable to do it back when the world was a wildly violent place; you'd be unable to function at all. Or imagine if you're one of the violent men who rules that world, how do square that with the love for your children? Our brains, IMHO, are far to complex for the reality of the world that we live in; we're self-aware but we're going to die, just that knowledge takes shocking levels of compartmentalization to deal with (and is likely the reason that religion came to exist, trying to deal with that obvious scism).
We're certainly not alone in our ability to do this in the animal kingdom, but we are, IMHO, alone in the complexity of our compartments. My cat (and just about all of them, for cat people out there) goes from the calmest, most lovable, stuffed animal level of compliance to a straight killing machine in <1sec if something catches his attention. And then right back to sleep once the threat or play opportunity passes. Now, in his instance, he's not doing it intentionally, but, the result is similar, a vicious hunter becomes a loving/wouldn't hurt a fly lap cat. I use this example because it describes my former cohort of cheaters well; doting fathers to their children, conscientious employees who would work 80 hours a week for as long as it took to get it done, "in love" with their wives from every outward appearance except.. Well, except that when you put them in certain situations, they would become vicious hunters. No lie was too low, no secret was safe from examination the next day.. Were they "better at" compartmentalizing than me? I don't really think so, I think they just used that ability to do bad things rather than not using it at all or using it for good.
That's why we have universal morals.
Beyond "don't kill people in your clan", I'm not sure I see much "universal moral" in human kind. You don't need to look too far back, even in just American history, before you see things that make your jaw drop viewed from where we are today. The "Mad Men" era with blatant cheating, discrimination against women, etc. And that wasn't that long ago, and we all thought that was "fine". Or go further back, when we all thought it was "fine" to enslave other people. Or go to other countries today, where millions of people think it's "fine" to sleep with children, have mistresses, or not let women show their faces in public.
Cheaters from what I can see, lack it compared to non-cheaters, but that is a generalization.
They have to, because, even for me who, given discussions on here with lots of people, might glean more pleasure out of sex than anyone else alive, AND have an admitted enjoyment of sex with new people, it's blatantly obvious to me, no matter how good the sex was with my AP, it would hurt my wife more than it would make me happy. The only way to make that a "good trade" is to say to yourself "My happiness matters more than her's" which, of course, is the antithesis of empathy. But I don't think that cheaters are all "broken" they are just doing what we humans excel at, compartmentalizing the way they feel about their spouse from the way they feel about their penis (or kibbles, or whatever it is they are getting from the A).
There's no way cheating gets to the prevalence level that it has if it requires two socio/psychopaths coming together to cheat. There's not enough of them to get to some high percentage of the population having either cheated or been cheated on. So, something else is going on here, people are "acting like" a sociopath without actually being a sociopath. How? Because many people, IMHO, most of us, have that innate capacity, we can "turn off" our empathy (granted, much easier for some than others).
Yes, many people can be conditioned to be soldiers, but not all. I've seen many recruits collapse from stress or have psychotic episodes because they could not take the pressure of training. Some human brains are not wired for combat.
And I think that the same could be said for cheaters. Sadly, I think that much like being a good soldier, it's something that takes "practice" to get really good at. How many stories do we see here of male cheats that can't perform with their AP? Now, of course, some of those are yarns being told to "protect" the BH, but, I am sure that some of them are true; and I think the analogy is "freezing on the battlefield". Most people can be "trained" out of it, some cannot. And some people never freeze at all and go right to "fight" (f**k).