It may seem a particularly Sayers' stance, reserved for overly intellectual women and readers who love Jane Austen. But Agatha Christie makes a similar argument in her short story "Magnolia Blossom."
In "Magnolia Blossom," a husband breaks the law by committing financial fraud. His wife was intending to leave him since, well, he's kind of a pompous crook. However, out of a sense of loyalty, she decides to stay. He then asks her to visit a man who holds papers that could possibly implicate him. Although she finds the task distasteful, she follows through. The man--Vincent--has a high opinion of the wife and gives her the papers as soon as she asks. When she returns home, she realizes that her husband not only assumed she would need to sleep with Vincent to get the papers, he sent her off to Vincent with that very maneuver in mind."You wanted to save your skin--save it at any cost--even at the cost of my honour," she says.
It is the final straw, and she leaves. Interesting enough, the final straw is not infidelity. (The husband has not always be faithful.) It is, rather, the absence of confidence, of being "straight" with her. And it is a good reminder that although women may not have a literary history of chivalry and knights pledging faith to each other, they yet have a literary history of honor within relationships.
Of course, determining when loss of faith has occurred is entirely up to the individual. There are no absolute rules in relationships.