I haven't seen season 3 yet but a huge part of Big Mouth is that the characters are all young kids just hitting puberty age who are totally and utterly confused by all the things happening to their bodies and those around them and all the new things they're feeling and experiencing. Many times the characters are wrong about things when they first encounter them because its the first time these characters have ever actually encountered such a thing and are exploring it. A lot of ignorant ideas are espoused, explored and often shot down or at least its ellucidated as to why they're negative, incorrect or offensive. This is a show where they have an entire episode spent to debunking myths behind Planned Parenthood and espousing the beneifts because an ignorant classmate thinks its about killing babies.
You're absolutely correct! ...but lots of the posters here don't think there's anything wrong with that character's statements themselves.