Sex that one will regret later. Can be a result of sex under the influence of alcohol or drugs, or sex with someone who has an STD. Other situations can result in regret sex, though, as long as the fornicator in question regrets the sex they had after they do it.
No one has regret sex on purpose. There is no fetish for regret sex.
Guy 1: Oh, look at Steve, he's going to the bedroom with Carla!
Guy 2: Carla is ugly, and has herpes!
Guy 1: Steve is about to have some SERIOUS regret sex!!