Revenge sex occurs when a person's significant other cheats on them. The person who was cheated on then through some means (usually facebook) finds out who the "other woman/man's" most recent ex was. The person then hunts down said ex and has sex with that person to get revenge on the cheaters.
Julian: Hey remember that big fat blob i was dating and cheated on?
Rick: yeah i remember
Julian: she had revenge sex with the other chick's ex-boyfriend!