Sometimes the default is what the reader knows the writer to be.

You're so right about that, yes.

And I also agree that sometimes that ambiguity is good in a character. But it seems like it almost has to be somehow pointed out that we're going to be ambiguous about this character for us to *get* that. Like, there's gatta be some clue that the author is telling the audience with a little wink or something that "hey, this character's gender isn't known" or "I'm intentionally being unclear about sexuality here". Otherwise, we do usually go into the default right away.