What place do women have in a young boys coming of age story? Well, Billy seems to be asking himself the same question. Where the Red Fern Grows is full of female characters—Billy has three younger sisters, fer cryin' out loud—but the only female character in the book who gets a name is a dog. (Well, except Grandma—Grandpa calls her "Nannie." Once.) Instead, we mostly get your stereotypical helpless and passive female characters. None of the women really do anything, aside from cry, giggle, and worry. So why is this theme so important? Billy learns to be a man in large part by distinguishing himself from women and women's work.
Because Where the Red Fern Grows is specifically about a boy's coming of age, it would be very difficult for girls to relate to this book.
To Billy, women aren't mysterious so much as they're just not interesting. He doesn't think there's anything worth knowing about the female world.