Read other letters about this article
Shouldn't the stigma of the fact that he's married -- and not to her -- fall on him? After all, she's not the one breaking marriage vows, so why should she be the one to carry the brunt of it in the way we speak of her? He is an adult. She is not obligated to be the enforcer of his supposed values. So why, when he lied and cheated, should we want to call her out of her name, but not him?
For my money, "paramour" does just fine for both of them.