Sigrid Undset, on a discussion she had with a married male friend who fell in love with a married woman; they both divorced their spouses and were about to marry each other when he discovered that she had been cheating on him on a large scale:
My friend was in despair, which was not to be wondered at, and he asked me from the bitterness of his heart:
"Tell me, Sigrid, do you believe at all that a woman *can* be true to a man?"
To which I made the decided answer: "No, I don't. I not only believe but know that a woman can be true till death, if she has an ideal which demands her fidelity. But true to a man--no, I don't believe any woman can be that."
And to put it mildly, it's an unreasonable thing to ask, I thought to myself, that a woman should be true to a man--seeing what men are. Or a man true to a woman, seeing what women are....
[Sigrid Undset, "Reply to a Parish Priest," Stages on the Road, Arthur Chater, tr., Christian Classics (Notre Dame, IN: 2012) pp. 186-187.]
I think this is quite relevant to the story of Kristin Lavransdatter.