There are at least a million articles both on- and offline trying to explain why cheating happens constantly, even though the vast majority of us agree that it is bad. Most of the explanations are complex and unconvincing. And then comes this gem of an article, and explains the phenomenon in a simple, no-nonsense, straighforward way.
The important shift it makes is to move away from the attempt to find a rational cause for the behaviour, and instead explain why it evolved. These are not the same things. There is still a “creation” concept hidden behind rational causes, because it assumes that evolved traits and behaviours need to make sense. But they don’t have to – they only need to provide an evolutionary advantage to survive and propagate.
In explaining how this exact condition – the mixture of both a desire to cheat and a desire to be faithful – was likely the one that provided the most offspring, the article explains it completely. That is the only sense it needs to make.
What is left is the knowledge that we will for the forseable future, have to deal with the dilemma.
-  It is actually a little more complex than that, as there are fitness indicators and “leftovers” that provide no advantage, but no disadvantage either, etc. – but the basic principle holds true. ↩