I work at a bookstore. I was cashiering today when a woman and her two kids (a boy and a girl, both somewhere between 13-15) came up to the register. The mom was buying 2 celeb gossip magazines, and the boy put down a book. The girl then walked up and set down the newest volume of the Sisterhood of the Traveling Pants series.
The mom says "You can't buy that."
Mom: Because it's too big.
Girl: [Brother] is buying a book that big. It's not very expensive.
Mom: [Brother] is a boy. You're a girl. And girls shouldn't read big books like that. It's too thick. Boys don't like girls who read thick books. You want boys to like you, don't you?
The girl went and put the book away.
ETA Livejournal-fu: This anecdote already immortalised in comic form.