When a female didn't wipe too well, then someone fucks her doggy style. As the ass is bouncin, the resulting shit stain on the male pelvis is a dirty Hershey kiss.
When you go shit in his mouth, he goes shit in yours, and you French kiss.
Also, when a guy goes down on a girl while she is on her period, and Gets all the blood in his mouth. And a girl has blood/cum in their mouth, they kiss each other.