Honestly I have yet to hear of a situation where the person getting cheated on didn't deserve to know. I mean, there's some truly heinous offenses going on here.

Some definitions, first:

Cheater- the one going outside the relationship

Cheatee- the one in the relationship who is being faithful

Accessory - the one commiting the cheating with the cheater, a.k.a the 'side piece'.

1. The cheater is letting the cheatee build their whole life on promises that the cheater knows they aren't keeping. Like, what happens when the cheatee buys a house with the cheater? Or decides to start a family? Keeping someone from the truth in such huge matters is evil.

2. The cheater is wasting the cheatee's time. You can get more money, find new friends, move to a new town. But you can never take back the time you give a person. When you cheat you take away the entire relationship from the other person. All that time, ruined by selfish choices. The woman in this story whose husband of 40 years is screwing around - even though it's not true, how could she keep herself from feeling that her life was wasted, loving a man who chose not to love her back? The cheater in this case just took a lifetime of memories, and poisoned those memories with doubt and resentment.

People who cheat on their partners are selfish cowards. You want to bang someone else? Fine. Break up, get a divorce, pay the price that ending a relationship demands. Own up to the fact that you don't want to keep your promises anymore. My dad told me one thing when I started dating: 'Break her heart if you must, but don't waste her time. If she's gonna have to get over you, the best thing you can do is let her start getting over you right away.'

Edit:typo