While having sex you finger the girls ass and make sure to get her poo on your finger. When ready, you pull out (before during or after cumming as it does not matter.) and smear her poo from one cheek, across her lips to the other cheek. When she gets mad at you, simply ask "Why So Serious?"
Bob: Hey did you fuck anyone last night?
Stan: Yeah bro, I hooked up with this one chick. Idk her name, but it didnt matter when I did the poop smearing