Flatworms do it like so

A really cool discovery made while doing some research for my marine paper: Penis Fencing (video)

Using new camera technology, Marine Biologist Leslie Newman of Australia's Southern Cross University participated in filming the marine flatworm Pseudobiceros hancockanus engaging in some odd reproductive behavior -- referred to as penis fencing.

During penis fencing, each flatworm tries to pierce the skin of the other using one of its penises. The first to succeed becomes the de facto male, delivering its sperm into the other, the de facto female. For the flatworms, this contest is serious business. Mating is a fight because the worm that assumes the female role then must expend considerable energy caring for the developing eggs.

It's one thing to read about this penis fencing business, but to finally see it in hi-res: So SO beyond cool.