That cheating is often going to be unrelated to happiness is news to no one who has been witness to infidelity as an uninvolved third party.
I'd venture that many people cheat because they will always be oriented to want to cheat. People who are not oriented in this manner tend to look for more complicated explanations. As they tend to project their differing mind onto the subject, and its requirement for a more complex rationale.
There are two kinds of 'why'. Why you did it in the first place, and why you keep doing it. The former is nebulous and has an element of chance. I think most of the intelligence is in the later. You can easily stop doing it after the first if it was an accident, a lapse in judgement, etc. Maybe it really was an accident, but then, they continue doing it. Why? The answer then is not mysterious at all; they learned how easily they can live with it, and how much they like the sex. Case closed? I think so.
It seems logically obvious to me that infidelity would be almost completely unrelated to happiness in a relationship because it doesn't matter how happy or unhappy I am, cheating is off limits.
To someone for whom that is not the case, why would it matter if they are happy or unhappy?
As the article states - being cheated "on" is not a matter of what you've done, someone who isn't inclined that way won't do it ever. I don't even put myself in situations in which the temptation would be there e.g. hanging out with members of the opposite sex.
In contrast, I've "stolen" carrier bags from the supermarket ever since the bag tax came in in the UK. The expense is trivial, I'd never even notice the difference. I just don't agree with the value system there, so wherever I can get away with it I do.
Some people feel that way about relationships. I think it's wrong and abhorrent, but then I feel like a lot of what goes on nowadays is pretty weird, so eh.
I'd venture that many people cheat because they will always be oriented to want to cheat. People who are not oriented in this manner tend to look for more complicated explanations. As they tend to project their differing mind onto the subject, and its requirement for a more complex rationale.