Moral of the myth: love triangles hardly ever turn out well. Not that you needed the story of Cephalus and Procris to tell you that. In fact, you may have already heard it in another Greek myth: the story of Glaucus and Scylla. Those two get tangled up with Circe the sea witch in a love triangle that leaves Scylla transformed into a monster.
Of course the Greeks weren't the only ones to fill their mythology with highly unfortunate love triangles. Let's take a look at a couple other prime examples:
King Arthur: you've probably heard of him. He was an awesome king who basically invented chivalry and recruited a bunch of noble knights to bring justice to England. Trouble came, though, when his best knight, Lancelot, and his gorgeous wife, Guinevere, fell in love. In many versions of the story, the affair that resulted ended up destroying Arthur, the Knights of the Round Table, and Camelot itself. Sound familiar?
P.S. For more on this tale, check out Le Morte D'Arthur by Sir Thomas Malory.
If you liked the King Arthur tale, you should definitely check out the myth of Tristan and Iseult, which plenty of people think of as its predecessor. In this one, King Mark's trusted nephew and awesome knight, Tristan, is sent to Ireland to bring Iseult back as a bride for Mark. Unfortunately for Mark, Iseult and Tristan accidentally drink a love potion on the way back to England and fall in love. Their secret passion ends up causing all kinds of problems—and in many versions, it ends up with all three leading players dead. Womp womp.