Dealing sensibly with a spouses sleeping around is not a pathology. Neither is dealing sensibly with a friend or colleague who participated.
What these tough guys don't realize is something which Margret Thatcher best put to words: being powerful is like being a lady; if you have to tell [or show] others you are, you aren't. (paraphrased, from memory)
We are discussing a situation as postulated. That postulate is a deep betrayal of trust and enormous emotional damage. Maybe _your_ reaction would be different, and those are your values, and that's fine. If I worked with you and knew your sexual mores were such that this was no big deal for you, my reaction could well be different.
But that isn't the case here.
"The boss has learned something about you and now he doesn't like you. Should the boss fire you?"
Like has nothing to do with it. Are you seriously suggesting that the matter under discussion is one of a boss not _liking_ an employee? "Oh, he slept with my wife, I really don't like that guy."
I'll take your question at face value and give you some examples.
If the boss doesn't like you because you are gay/introverted/unsocial/not into sports/don't make a good drinking partner/refuse to perform sexual favors - no, the boss shouldn't fire you.
If you slept with the boss's wife or a co-worker's wife, then you have no place on a team and yes, you need to be fired.