sex proxy
One with whom your sexual partner has had sexual relations with.
After his girlfriend confessed all to him, my buddy Hans found out he has a dog for a sex proxy!
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.
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.