Ping Pong is slang for three people having sex with the person in the middle going back and forth. Pizza is slang for underage sex in paedo circles. The artwork from Comet Ping Pong is on display above, but it was the comments on the owners Instagram feed that were the creepiest. James Alefantis has made his account private as have all the creepy commenters but they've been archived here. The Reddit post has been taken down but the research is now available over here.
I've uploaded James Alefantis Instagram feed now he's made it private.