flatworm mating rituals are bizarre. according to the hutchinson educational encyclopædia:

In many flatworm species, the sperm is not received into a genital opening, instead the penis pierces the skin to inject sperm straight into the body. As this causes some damage to the flatworm, in each sexual encounter both flatworms attempt to take the male role. Consequently flatworms will vie with each other for an hour or more with each attempting to stab the other with its penis, whilst avoiding being stabbed itself.