This video. Now.
That video from the excellent series The Shape of Life shows the penis fencing behaviour of a species of marine flatworm. The loser is stabbed and fertilizer by the winner’s penis (they are hermaphroditic, but they fight because being the “mother” means spending a lot of energy and resources in producing the offspring). Although this is a particoularly spectacularly bizarre ritual, is not the only case of hypodermic insemination – the mating behaviour in which the male pierces the female’s skin with its penis and injects sperm directly into her body cavity. Rotifers of the genus Brachionus and many other invertebrates do it. The most famous and studied are the bedbugs, Cimex lectularius. Here’s a video interpretation of their mating practice. Although I’m not sure I’m ready for a human reinterpretation of the flatworm penis fencing.