Oh Kotaku. You're rapidly becoming the Daily Mail of the gaming world.
For those allergic to Patricia Hernandez, TL;DR: Mod menu allows players to cycle through animations. One of them is a sex animation from the Poppy Mitchell mission. Using this animation allows for another player to be "poppy"
But the dog beat me over the fence. http://faqsmedia.ign.com/faqs/image/article/111/1116164/skull_iwhbyd.jpg