I believe our perception of the fathomable can grow as we grow. But as we are bound to our senses and eventually by our reality, the fathomable will also be bound. We can reach out and try to understand within those boundaries. But to go beyond is impossible by its very definition. The fathomable will always be a limited part of everything, mirrored by an unending sea of the unfathomable. — Mind Dough

I agree.

You say " I have involuntarily become aware of spouses who have cheated on their partners--both are completely separate cases", and you ask a question about duty to inform.
I agree with Galuchat, but will offer a something different, perhaps more colloquial, argument.

What I have found so far with schools of moral philosophy so far is that they don't often seem to provide specific guidance for real-world situations such as yours.

However, perhaps we can take this in steps:

1. Is there a legal responsibility to inform? [this does exist in some cases where you may inadvertently learn something].
-In the situations you describe, the answer is 'no' (in most or all jurisdictions)

2. If you do not inform imminently, is anyone going to die?
-No
[you can ask similar questions to this for really bad outcomes other than death, but I think for most of those the answer would still be no]

3. Is it clear that if you do inform you will create a net good (this is a utilitarian sort point, and can be stated more formally than by using 'net good', but here I am abbreviating a bit)?-
-I think it's unclear. I don't believe the reason you cite " I would want to know the truth" is by itself sufficient, because your wanting something doesn't give it moral authority. On the other hand, you may be justifying this from a 'do on to others, as you would want done onto you' perspective. Even there, you may want to ask yourself, would you really want to know under all circumstances?

You may be getting the sense that I am guiding you a certain way; I am not. If your gut (yeah, I said 'gut') tells you to reveal, maybe its the right thing. But as you say, there are likely to be consequences you may not want.