Ok if we're talking extreme examples the only ones off the top of my head that I can think of where it would be ok not to tell an exisitng partner about an affair was if that partner was in a coma or suffering from advanced dementia. In those cases you really couldn't have any kind of meaningful conversation with them. Otherwise out of respect there is always another option.
Everything will be ok in the end. If it's not ok it's not the end.