I find it completely believable that upper class girls knew nothing of sex. Unlike the lower classes, they weren't living in small homes with thin walls and potential exposure to their parents' nocturnal activities, or to women giving birth nearby or nursing babies in view of others, or having to help with the farm animals. They lived in mansions with governesses and constant supervision. Any saucy activities or discussions could be kept in private rooms far from the eyes and ears of children and unmarried women. They didn't go to school with all manner of people sharing information. They were educated by their governesses and tutors in only "appropriate" subjects, which didn't include much anatomy. Their media was very limited, to approved books etc. All the adults and boys were strictly trained to keep any frank talk or salacious materials well away from proper young ladies. Their friends were carefully limited to girls who were sheltered in the same way. They had periods, but dealt with them with rags etc., no need to touch. They would likely have been taught to avoid touching their nethers, although they would have been told it was because it was dirty, being where pee and poop come out. No suggestion of possible titillation, and why would most of them think to try? The appeal of marriage was that you had status and security and decent companionship if you were lucky, and could have children, which just come naturally after you're married. They were conditioned from birth to set their boundaries of what was acceptable to think about or talk about so far back from actual sex that they would likely not have wondered up to that point what was that far past the boundary. Unless they were particularly inquisitive, like Eloise, and also not easily cowed by chastisement.
