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.