So I should be upset if my DD ends up being a ballerina?!
Nobody said this.
Look at it this way: how many adverts, toys, clothes, TV programmes, books, whatever, is your daughter going to see linking "ballet dancer" to girls? Now, how many is your son going to see showing that little boys can do ballet too?
And while you're adding up the many, many occasions in which I'm sure you've seen ballet imagery targeted to little boys, think of it this way, too: this advert is showing babies, doing basically the same thing, learning to stand up and learning how to use their arms and legs. Why is the girl baby seen as being a potential ballerina for doing this, while the boy is seen as being a potential mountain climber?
There was a pregnancy week-by-week site I saw when I was pregnant, that talked about the point where your developing baby was developing limb buds. It said something like: "if your baby is a girl, she'll be doing ballet twirls in no time! And if you're expecting a little boy, it won't be long before those legs are booting a football into the neighbour's garden!"
Foetuses. With limb buds. You don't even get to develop proper limbs before you're told what your acceptable hobbies are going to be.