All said and done, the concept that people should be monogamous isn’t natural. Look at the animal kingdom. It’s pretty rare.
Some people just don’t fit that mold. But society tends to look down on them. So for some, bucking the system is probably a source of joy and achievement that they want to share. For the listeners, humans often go along with things to avoid awkward interactions. Then go home later and say to someone, “I can’t believe she just said that.”

Seriously, I think that fits. Many people (not all) cheat because they are unfulfilled in life. And the same cohort also might go to class reunions for the same reason… to try to find fulfillment.