NC of course.
The subject title is provocative but largely sums it up. Is there ever a situation where not disclosing an affair can be justified, at all?
It's an open question but for example (and this doesn't necessarily completely describe a place I am in (though close)):
A married person and a single person have an emotional (but not physical) affair over several months. It's weak, it's wrong, and although they didn't stop before their feelings got too far, they both eventually realised it is wrong to continue on a basis of dishonesty. Both agree to stop contacting each other. The single person moves on, and the married person resolves to commit to their marriage and invest the energy and commitment that they realise (all too late) they should have thrown into their marriage in the first place.
The affair has introduced something dangerous and unstable into the marriage (which the innocent party does not know), but it has happened. If there is something that can be salvaged from the marriage, is it always 'better' for the guilty party to tell their spouse? Why would they do it - for honesty, to try and regain a bedrock for the relationship, or (sometimes) to simply avoid living with the hidden guilt? Perhaps that is the price to pay for the guilty party - to carry that burden and to not offload it?
Or, simply, if the innocent party loves their partner and is entirely happy in the marriage (though now based partly on a lie), would they actually (in every circumstance) wish to know?