The sex life of the marine flatworm can be summed up in two words—penis battle. (And you thought those only happened during office meetings.)
The duel, which can last for up to an hour, is all about not getting knocked up. Both flatworms start off hermaphroditic and use their sharp penises as swords to stab and try to impregnate the other first. The loser gets stuck with the kids and the winner gets to scoot off like the original deadbeat dad.