May we still refer to such a woman as a side piece?
Q: Getting any on the side?
A: It’s been so long I didn’t know they moved it.
How about concubine?
I would prefer she be known as an adulteress because that’s what she is/was and knowingly slept with a married man without considering there is a wife and kids who would be the ones who have deep wounds from this. Of course, the man is just as guilty, too.
Might I suggest Monogamous Whore? Then it would be suitable for Women, men and alphabetsoupers.
“May we still refer to such a woman as a side piece?”
In “The Age of Innocence” by Edith Wharton a mistress is referred to as an ‘alternate establishment’.