A mark left on someone's flesh resulting from being slapped by a penis. The impact of thebulbous glans penis and narrower shaft leaves a mark with a shape much like that of the fruiting body of a mushroom.
Your mother wasn't very careful with her teeth when she was sucking my wang, so I pulled my dong out of her mouth and beat her on the face with it, leaving several mushroom-shaped welts that developed into bruises that were clearly visible when she picked you up from school later that day.
When a man is receiving oral sex, he takes his (substantial) scrotum and flattens it all over the fellator/fellatrix's face. Much like tea bagging, but pressing the scrotum into the face, not putting it into the mouth.