Cheating on someone without them knowing about it reduces their eudaimonia even if it doesn't reduce their subjective happiness. It means that their trust in their partner is misplaced. It damages their honor, because they are living a lie and yet unaware. It means that every time the cheater looks into the eyes of the partner they've cheated on, they're aware of that fact that they have broken a promise and continue to hide it. Basically, it makes a mockery out of the person who was cheated on, even if there never are consequences that they find out about.
When you think about the terms you used for example, their honor, them being made a mockery of, isn't the value of these things only related to what we attribute to them?
In other words, if the person who broke the promise, does not see their partner affected at all in these terms, and the person he slept with doesn't even know that their is a partner, in what frame is their honor for example affected?
The person who broke the promise still views their partner in the same way, the partner still view themselves in the same way, all the people around view it in the same way. In what terms has it been affected then?
At the same time, i think people have a right to know the truth regardless, i can't explain why though. Because unlike the example Kathryn used much earlier, about a tree falling in the woods, i fail to see the value of these things if we don't attach them to people. In the tree's case it did fall, and it did make a sound. In this case, aren't those things only happening if it is recognized by someone?
When acting a certain way, people can ask their self, "is this the best possible expression of who I am?" Is cheating on a partner, and betraying their trust even if they don't know about it a good expression to put forth?
I'd like to use this to add something. I don't view every action in terms of right and wrong only. Somethings are neither. In some situations, i believe its best to do something for example, but that doesn't mean you'd be wrong if you didn't.
When i apply that to this issue its like this. Cheating is always wrong. Not following through with the promise (either because it no longer applies or because the grounds change) and sleeping with someone else without telling your partner isn't always wrong. Most of the time it is though, but in some situations its justified. Few extreme situations. In some of those situations, it might be best to still not do that, but if you did that doesn't necessarily mean that you're wrong (or right).