Not to minimize the pain and suffering caused by infidelity, but payback is not what karma is all about. Unfortunately, in the west the term 'karma' has been turned into a synonym for 'what goes around, comes around' and this is an extreme oversimplification.
Karma is much more complex than the simple idea as you state in your question.
Karma is the result of thought, speech, or action and can be manifest as positive, negative or neutral. Karma means "action", and in simple terms is a chain of events arising from certain causes and conditions, which due to other causes and conditions remain in continual state of progression.
By acting (thinking, speaking, doing) in a positive manner, one creates or rather promotes positive karma onto other beings, which then further those positive actions on and on - creating loving kindness, genuine compassion, tolerance, non-judgment, many other good qualities.
Likewise negative karma can manifest in the same way.
This is just a brief summary of karma and hopefully it helps to give you a beginning. Actions can be the result of many related and unrelated causes and conditions, and karma can manifest in relation to the significance of those causes and conditions, as well. For example, intention counts - if one were to cause harm to another being by accident or by lack of intent it does not carry the same weight as with maliciousness.