It's bad to sleep with your friend's ex.
Jerry has been abstinent for a long time and finds out his friend just broke up with his attractive girlfriend.
Jerry wants to have a sexual encounter.
Jerry goes onto a dating website and meets a nice girl online.
Jerry meets up with his friend's ex and ends up hooking up with her.
Jerry's friend meets the girl, and they get along well adding another person to their circle of friends.
Jerry's friend finds out, and he loses a lot of trust in him.