The American west was once very lopsided as well, males became cowboys and the women rarely traveled west. In one generation the few women out west had equal numbers of both gender kids and now that history is not important to the culture.

The book is interesting, but it was written for a society that doesn't exist. Women and men will not be as lopsided in a colony. The first few ships might be mostly male construction workers, but after that expect to see a much closer balance.