Okay, here's the thing:
Kids do this. At some point, usually starts in preschool or around kindergarten, we start hearing about 'boy' and 'girl' things.
This could be colors, activities, toys, clothes, etc etc etc.
This is how simple children are creating gender identification. They are not trying to be neanderthals, they are just circulating the popular beliefs of the playground population. Kids are still young, they like definitive things, not shades of gray.
My advice is not to worry. The best way to teach our children is through our own example, period. We can flap our jaws all we want about 'boys can do anything, girls can do anything' but our objections don't matter. Our actions do.
They do grow out of it, by the way. I've known quite a few youngsters who most certainly, with all seriousness, espoused those opinions when they were four and five and six and so on. They are KIDS. Correct it matter-of-factly ("oh, plenty of women wear pants and do you want to see something? Let's look at how men dress around the world. Introducing the kilt!") Use it as an opportunity to share your excitement of how neat it is that all the people of the world can do lots of different things.