Taking a girl home, proceeding to bang her but then when your maybe half way through, she decides to quit cos shes had enough fun.
"Cory takes Morgan home to finger her with his penis, Morgan decides shes had enough fun and tells him, "I'm done". Cory is left there with his unfinished business. This explains fraction sex.
In a threesome with two guys and one girl. One guy laying on his back fucking the girl in the vagina simultaneously another guy fucking her in the asshole. The balls rub together causing friction.