Sorry, Penis Fencing isn't the immature behavior of drunk frat boys, something curious 8 year olds enjoy, or the latest film by Ron Jeremy.
It's the mating behavior of a certain species of flatworm.
This species is hermaphroditic (both lady & man parts) so they battle with their pricks to see who the father will be.
The Flatworms "fence" using two-headed dagger-like penises which are pointed, and white in color. The "winner" is the organism that inseminates the other. The sperm is absorbed through pores in the skin, causing fertilization in the "loser".
As you'll see from this video "It's known as Penis Fencing. And the flatworms are the swordsmen."