Part of it goes down to when children are young, we create constructs around what's expected of them as they age. Look at toys, the girls have toys covered in pink, tend to be somewhat frillier, and designed towards concepts of friendship and cooperation; boys have toys that are geared towards power and conflict (Transformers, G.I. Joe, and so on, from when I was a kid). Then we know there are boys who gravitate towards playing with Barbie dolls, and girls who want the G.I. Joe stuff, which people with more time and thought on this subject than I have state it's a sign of their gender not syncing with what we expect of their wang/vagina equipment.