Coupla thoughts .. and I'm going to quote a couple of people. IN the interests of full disclosure, my first relationship outside my marriage started as an affair. I did eventually tell my husband about it and my relationship with this person continued ... my husband and I separated for a while and then got back together. But ... ultimately, it was an affair. So that said:
If you are the cheatee, I don't believe that you are the cause, but a symptom of a problem that has nothing to do with you.
I cannot express how strongly I disagree with this. I was the cheater. My husband was the cheatee. The reason I cheated on him was that in 10 years of marriage, we had not had sex in the last 5 - not because I didn't want it, but because he didn't. He has different sexual drives than I do, different desires, and ultimately he rejected me over and over an over and over again until I quit asking. We went to marriage counseling. I begged him to see a doctor. Etc. Etc. Etc. Five years later a flirtation turned to more ... and I realized what I'd suppressed and been missing in my life. To say that the reason I cheated had "nothing to do with" the cheatee (my husband) is patently ludicrious. And I know that I'm not the only person - male or female - who has been in the same position. And whether it's sex, affection, romance, whatever .. to say that the "cheatee" has nothign to do with the problem is ... quite simply ignorant. It takes two ... cheating doesn't happen in a vacuum where there is one completely evil wrong person and one innocent blameless victim.
I'm not saying cheating is right. I'm not saying what I did was right. But the "cheatee" was not innocent or blameless either.
From the OP:
in reality (as long as it stayed under the radar) it was highly beneficial to everyone.
That's completely and totally justification. How is it beneficial to everyone to hide and lie and cheat someone you love? And I say that as someone who did all the above for what I thought was a valid reason - it was beneficial, my husband is happy being married to me, I can get what I want elsewhere and stay with him. I'm happy with him other than sex. No one has to be hurt. Etc. I totally told myself that it was "highly beneficial to everyone" involved. That's a lie you tell yourself to justify lying and cheating to someone else.
Period, the end.
I realise that my stance is probably not the popular one here, but it's very much how I feel .. having been there myself.
Does it happen? Sure. Would I do it again? I honestly don't think so. The fallout from the finding out (on all sides) was devastating. It destroyed relationships. It destroyed trust. It was the worst thing that has ever happened in my life. I accept full responsiblity for my role and actions in it as well.
Justification is exactly that ... and if you're honest with yourself you'll admit that the truth is that it's only "beneficial" in the sense that you/your partners are "getting away" with something.