You can call bullshit on me, I'm too lazy to go look for the reference, but once I read somewhere (probably here on /.) that before the 80's, enrollment for maths and CS majors were growing more or less equally for men and women. Then, in the 80's, around the time the movie industry created this idea that computer experts and hackers were all-male and toy computer manufacturers followed suit, enrollment for women plunged and has never recovered ever since.
According to that, the idea that computer science is for boys was a social artifact of gender differentiation for toys in the 80's.