You shouldn't cheat on your fiancee.
Bob is engaged to his high school sweetheart but meets a very attractive girl one evening.
Bob wants to feel like he is attractive to women.
Bob goes back home to his fiancee after charming the attractive girl by being funny and sweet.
Bob flirts with the new girl who responds in kind, leading to the two sharing a night of passion.
His fiancee is happy to hear that he had a great evening.
His fiancee finds out and is devastated.