Flatworms

For flatworms, sex is more like war than love. Like all sea slugs, flatworms are hermaphrodites (they have both male and female sexual organs). In this case, the male organ turns out to be two dagger-like penises that they use to hunt as well as mate. During mating, two flatworms fight (i.e. "penis fence") to stab each other, while avoiding getting stabbed.