A very good question ...

... But if she was not a lesbian?

Perhaps she lived with Lewis precisely to avoid “sensual disturbance,” and to put all her creative energies into her writing instead.

And suppose she had a really lively imagination?
A friend of mine who is a very good short story writer - but who I will not name because the conversation was private - told me once how exasperated he became when people assumed that his stories were always in some way autobiographical. "They just don't seem to understand that I make these things up."
David 's post is excellent.