Here's a video showing flatworm's penis fencing. Flatworms have both male and female sex organs and they fight to determine who gets to be the male and who gets to be the female. The winner of the fight is the flatworm that is able to pierce the other flatworm with one of its penises. You can read a little more about it here on PBS.org.
During penis fencing, each flatworm tries to pierce the skin of the other using one of its penises. The first to succeed becomes the de facto male, delivering its sperm into the other, the de facto female. For the flatworms, this contest is serious business. Mating is a fight because the worm that assumes the female role then must expend considerable energy caring for the developing eggs.
