Single Guy With "Mistress" ? [Reaper]

This was inspired by an episode of Reaper.
In the episode, someone referred a woman that the Devil had been romantically involved with [but had not married] for years as his “mistress”.
I’d always been of the belief that a man’s girlfriend wouldn’t count as a mistress unless he was married.
Am I missing a turn of the phrase?
Is the Devil some kind of linguistic exception?

Nope. The more old-fashioned meaning of Mistress is “kept woman.” Or just a single woman in a long-standing sexual relationship with a man to whom she was not married.

Back then, a “girl friend” was somebody you took to the sock hop; maybe you sometimes shared a malt.