As hermaphrodites, banana slugs having sex gets a little complicated. Both parties have penises--large, body-length penises--that have to be inserted into each other's female tract. And then things can get a little sticky. Sometimes, it seems that one banana slug is stuck in the other, so it chews off the other guy/gal's stuck penis (video) before they can go their separate ways. That's one theory.
Apophallation, or amputation of the penis, is still a somewhat mysterious behavior in banana slugs. An alternative theory suggests they're not stuck at all: it's plain old selfish behavior explained by sperm competition. By chewing off the other slug's penis, the chewer insures that its partner is unlikely to mate again. (Sure, the once-hermaphrodite chewee slug can still mate as a female, but a banana slug missing half its sexual organs is less desirable.) That ensures that all of the chewee's eggs were fertilized by chewer, who is free to mate with more slugs and spread its sperm farther and wider.