sex by proxy
This occurs when someone with whom you have recently had sex also recently had sex previously with a "third party".  You are then said to have had sex with third party by proxy.
Jim: Hey, Ted, you know that Julie chick you were bangin' last night?  Ted: "Yeah"
Jim: Man, she has been doing that nasty dred locked gansta, Willie, for a month.
Ted: So?
Jim:  Ah Man, you had sex by proxy with the dred man.