* Cheating is bad.
* Therefore, Cheaters are bad people.
* But I’m not a bad person.
* Therefore, I am not a cheater.
* Therefore, whatever I did wasn’t really cheating.
* Since I wasn’t the one who broke the rules, it must have been you.
* Therefore, you caused the affair when you (insert some failure here).
* Hiding the affair from you was my right, and you deserve to be fooled because it is your fault I had an affair.
* Besides, lying is bad and I’m not a bad person, so I wasn’t really lying when…