I have Phoebe and Digger but all the books for boys seem to say "if you wear a dress you are a boy".
TBF my boy doesn't really want to wear a dress all the time (a tutu for dancing is OK) and it's probably more about it being OK to wear pink (which he does like, he doesn't think it means he's a girl, but he STILL says "it's for girls").
But he is dangerously close to "boys are stronger than girls and boys fight/hit".