Earthworm sex involves some very close cuddling. The couple slithers up right against one another but facing opposite directions. They align their citella—the thick ring containing their reproductive bits—and secrete slime to envelop each other. Then they get it on. From KQED Deep Look:
The earthworms embrace by using flaps on their clitella, and exchange sperm that travels outside their bodies. As they snuggle, the sperm flows into internal storage sacs. After mating, an earthworm produces a sheath with its clitellum that it shimmies down its body. The protein-rich ring moves over tiny holes, where it gathers eggs and some of the collected sperm. Then, the ring slips off the worm to become a cocoon with one or more developing earthworms inside.