People treat little kids differently based on whether they are a boy or a girl, regardless of what you as a parent want (or even what the kid wants).
I'd love to hear an example of that. Remember, I'm talking about kids who aren't even 1 year old yet.
Sure. What do kids get as a gifts for (insert holiday celebration of your choice) from their relatives (especially Grandma/Grandpa)? Do the girls all get Bob the builder toys and the boys get a play kitchen and dolls? Probably not.
Your "wearing pink" example is silly. How would a 9-month-old know that pink is a "girl" color?
How do you know that pink is a "girl" color? It's societal. Society thinks that pink is a "girl" color, so people tend to give girls pink clothes. The girls then either like the color or recognize that they get positive reinforcement for liking it (e.g. grandma smiles and says "How cute!") and wear it more often.