It all depends on the circumstances and outcomes that the facts revealed could possibly create. For example, if telling the truth gave a better outcome, then the truth should be revealed, however if it caused large commotion or restlessness or even conflict that would take effort to solve, then no. It all tends to depend on the characteristics of that truth.
It's obviously beneficial to my hypothetical wife for me not to know, but not for me. For me, it's beneficial to know that my hypothetical wife is cheating on me. As I said, different facts different "rules". It's a very difficult question, and depends entirely upon the affected's view point.
That's a good point. It's very difficult to judge which path is more beneficial. Sometimes it's obvious, sometimes it's not obvious.
And you're right, I failed to consider whether it was a recent fling, or a past fling. Either way, I'd rather know than not know. Even still, there are so many factors to consider.
I cannot give a very good answer to this hypothetical situation, as I have never been married (early days yet), been in a serious relationship or been cheated on. So whatever answer I give you wouldn't be accurate to me, since I have never been in such a situation. So all answers I could possibly reply with would be hypothetical and probably too "calm" of a response to the situation.