The OP: "every time I see his face, I feel like that just kills me inside."
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.
What if the boss had said, "I just found out a favorite employee is gay. Now every time I see him, I'm terribly upset because I know he's going to burn in hell. I feel so betrayed that someone I trusted is so immoral. Should I fire him?"
The two aren't even remotely related. Being gay is a strictly individual choice that has no effect on the employee's performance or that of other employees. Sleeping with the spouse of a co-worker immediately and intimately involves the co-worker and is devastating to them. I don't even begin to see how you could compare the two.
"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.
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.