this really bugs me.
I have 2 dds, and one ds (a baby still).
dd2, as a toddler/young child was loud, demanding, loved racing around, favourite colour red, and everything bright/noisy. none of this pastel malarkey, or sitting still doing puzzles for her. she did wear dresses out of preference (have you any idea how hard it is to find bright red dresses? pink was hated, as were flowers) - this was a sensory issue as she didn't like the feel of trousers around her waist.
then she went to preschool.
and was repeatedly told her clothes were 'too pretty' to spoil painting/playing outside; was told by the girls she must be a boy (she had shortish hair, and liked doing 'boys games'). told by the boys she couldn't be a girl because she didn't like pink/didn't play with dolls etc.
she was really quite unhppy for a while - she couldn't understand why she was being told she wasn't a girl, when she knew she was. she had me telling her everyday that I didn't care what happened to her clothes (in terms of paint spills etc), but was told differently by the person in charge at school. she got the message she was meant to take more care if she was wearing something 'pretty' than if she was wearing a t shirt and jeans.
she started liking pink to fit in ith her peers - she still prefers red and brighter colours when she is being honest with herself, and I hope that one day she will be strong enough to be honest with her friends too.
all this came from school, and other people's expectations of her. she had no notion that she was not 'supposed' to like boys games before she started school. if anything, she probably had rather more 'boys toys' than 'girls toys' at home. she certainly had never been told not to run about/climb trees/paint/glue/whatever because she was wearing a dress 
I have repeatedly had words with her teachers. there is only so much they can do, of course, since a lot of the attitude is also coming from the other children.
but it has been truly eye opening as to how much pressure and expectations are put upon very small children.
now I have ds too. people repeatedly ask me if being a mum to a boy is any different from being a mum to girls - ds is 7 motnhs old! so far, he has slept not a lot (like most small babies), drunk a lot of milk (like all small babies), babbled a bit (like all small babies), and weed and pooed a lot (like all small babies) - where exactly is this difference supposed to come into play? and yet there are people at every turn desperate to put any slight (perceived) difference down to the fact that he is a boy.
he is at the lovely piercing shriek stage - I have lost count of the number of times people have said he is being loud because he is a boy - if that is the case, how come both my girls also had the same loud piercing shriek phase (along with most babies I have come across)?
I do not understand this desperate need to fit everythign into neat little boxes, especially since the labels for those boxes are social contructions in the first place.