Anne's mother advised her not to discuss sex with boys and not to answer them if they brought up sex-related matters. This suggests that Mrs. Frank assumed boys would discuss sex with girls in a joking or lewd way.
At the time, Anne agreed with her mother wholeheartedly. She had noticed that even her parents and girlfriends were "mysterious or obnoxious" when they talked about sex. So she assumed sex-related discussions between boys and girls would be intolerably awkward or disgusting.
But when Anne tells Peter his cat can't be male, Peter invites her to come and see for herself. His manner is so calm and polite that she ignores her mother's warning and accepts his offer.
When Anne comes to "have a look," Peter turns the cat over on its back and points out its "male sexual organ." He speaks in a straightforward tone, and he continues talking about the cat's sexuality "in a normal voice."
Peter's matter-of-fact manner puts Anne at ease and gives her the courage to ask him what the "male sexual organ" is called. He says he doesn't know but will ask his parents.
After describing this incident, Anne writes that she had never discussed sex with another girl so normally. She is sure her mother wasn't trying to warn her not to talk about sexual matters with a boy who could discuss them in such a frank, natural way.
Mrs. Frank's advice and Anne's discussion with Peter appear in Anne's diary entry for Monday, January 24, 1944.