It's interesting, as I have this conversation with my kids - ie actually let them know, there's actually no good reason for this, it's just a societal rule. However, I then try to explain that it is often advantageous to follow these rules even if it doesn't seem to make any sense, because of the advantages you can get by being cohesive in society. It's a good exercise, and also helps me think, there really are some dumb rules, and the calculus is weighing up when to disregard them.