Penis fencing! aka Raiders fan brohugs...
Does it look like these flatworms are dancing? Well, not quite. They're actually penis fencing.
This species is hermaphroditic and all individuals have working penises, testes and ovaries. Typically, being a mother is more time consuming and "costly" for the flatworm - eggs are larger and require more energy to form than sperm do. So, neither wants to be the mom. They fight with their penises, and the loser is inseminated by the winner.