http://www.refinery29.com/2014/07/71675/sex-slang-history?utm_source=sms

Forgive me if this has been brought up before (haven't seen it if it has), but now that pizzagate is a thing and I see references literally everywhere (Wtf SNL? http://www.nbc.com/saturday-night-live/video/pizza-town/3457935?snl=1)) and I started looking into pizza-related themes (have you looked up "pizzacore"? - plenty of interesting people line that "genre"...), I also did a very cursory review of a few terms.

Ping pong - seems to have been some confusion (?) on here about what it is but seems pretty clear in Internet land that it refers to a sex act involving 3 people where the person in the middle gets it simultaneously from the front and the back

Also Pizza - seems like it pretty clearly means vagina, not necessarily underage (hence the cheese meaning "child").