Sea slugs. You wouldn't expect them to feature in Naughty Nature would you? But sea slugs have an interesting mating process, which can vary. I haven't been able to find what causes the variations but that would be a fun research project.
Individual sea slugs can be male, female or hermaphrodite. Mating can involve two animals, a male and a female, or two hermaphrodites, or anything up to 20 individuals (lined up) who can be any combination of male, female or hermaphrodite, where they can pass sperm to the animal in front of them in the line.
Puts a new spin on line dancing, huh?