"Genteel women suppose that those things do not really exist about which it is impossible to talk in polite company"

Friedrich Nietzsche is slamming the idea that "genteel ladies," who are usually viewed as fine-tuned and cultured, believe that any topic that can not be discussed in courteous business does not really exist. He is suggesting that this belief is narrow-minded and neglects the true intricacies of reality. He might be indicating that these females are too concerned with social appearances and good manners, instead of engaging with challenging ideas and problems.