Because flatworms are hermaphroditic, capable of being the male or female during sexual reproduction, the first rule in mating for any pair of these lovers is to decide who plays what role. To settle it, they engage in an activity that has been described as "penis fencing," which is pretty much what it sounds like. Using their penises as swords, the two flatworms battle it out to see who can inseminate the other first, which is accomplished by stabbing the other in the underside. The loser ends up being the mother.
These battles are hardly innocent foreplay. Flatworms will often gouge gaping holes in one another, and the fight can last for up to an hour. Not only does the winner get to gloat, but it also gets to swim off without any further biological responsibility in producing the offspring.