Very few people who discourage their son from wearing a dress or taking up ballet do so in an attempt to enforce some grand imperative of how things should be. Rather, parents encourage certain traits because they believe that these are the traits that will equip their children with the best odds of success. And they're right- a boy who enjoys t-shirts and sports is at a tremendous social advantage over one prefers dresses and ballet. It has nothing to do with dictating how things should be, and everything to do with trying to maximize a child's expected future happiness given how things are.

"And they're right- a boy who enjoys t-shirts and sports is at a tremendous social advantage over one prefers dresses and ballet"

Perhaps in some very conservative societies.

Where I live I'm confident that a boy who enthusiastically engages in things which actually interest him is at a social advantage over boys who halfheartedly pretend to enjoy in whatever his parents think is normal.

Ballet, for sure. Maybe not dresses so much, at least not if he is doing the wearing, but having an interest in the clothes of his female peers certainly isn't any kind of social problem.

What about a kilt?

People would think you look like a hipster.