From the Florida Keys National Marine Sanctuary Facebook page, here are two lobsters doing it — apparently in the missionary position.
But looks can be misleading. What's actually going on here is external fertilization — that is to say, procreation without any of the potentially awkward penetration. Male lobsters produce spermatophores, packets of sperm, which they attach to the female's sternum. That's what you see happening here, according to the Sanctuary. Later, the female will use that sperm to fertilize her eggs.
Thanks to David Shiffman and Carin Bondar!