1. His wife cheated on him; that’s why they got divorced.2. She was having an affair with a married man.
 cheat on somebody have an affair這是最普遍的說法.
他太太有外遇, 所以他們才離婚. 她與有婦之夫外遇.< >
I just heard that the girl I am obessed with is going to get married.