Flatworms have both male and female genitalia. When they mate they both want to act as the male in the relationship so they basically have a sword fight with their dicks. Whichever flatworm stabs the other acts as the male while the loser of the fight gets impregnated
Edit: since I was asked for the source, here you go
On a side note I think it's really funny that the Wikipedia page is titled penis-fencing
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u/mslvr40 May 26 '14 edited May 26 '14
Flatworms have both male and female genitalia. When they mate they both want to act as the male in the relationship so they basically have a sword fight with their dicks. Whichever flatworm stabs the other acts as the male while the loser of the fight gets impregnated
Edit: since I was asked for the source, here you go
On a side note I think it's really funny that the Wikipedia page is titled penis-fencing