Dan Wohl at The Mary Sue examines the idea that actually existing historical sexism can excuse sexism in fantasy fiction, on which Tansy Rayner Roberts elaborates:
History is not a long series of centuries in which men did all the interesting/important things and women stayed home and twiddled their thumbs in between pushing out babies, making soup and dying in childbirth.
History is actually a long series of centuries of men writing down what they thought was important and interesting, and FORGETTING TO WRITE ABOUT WOMEN. It’s also a long series of centuries of women’s work and women’s writing being actively denigrated by men. Writings were destroyed, contributions were downplayed, and women were actively oppressed against, absolutely.
But the forgetting part is vitally important. Most historians and other writers of what we now consider “primary sources” simply didn’t think about women and their contribution to society. They took it for granted, except when that contribution or its lack directly affected men.
A good example of that last point can be found in Kimberly Klimek’s dissertation Forgetting the Weakness of Her Sex and a Woman’s Softness: Historians of the Anglo-Norman World and their Female Subjects, which looks at a particular period in history often used as inspiration for fantasy novels, which was particularly rich both in historians and powerful women both.