As they get older girls are told that some jobs are not for them, that they should be working to get good husbands. TV says to look pretty. [...] So by the time it comes to pick out a career or major in college these days, the number of women choosing computing, mathematics, or engineering is small (and in my experience much smaller than it used to be).
Granting all of the above is true, it's still not clear what can be done when many/most women, whether for reasons inherent or socially acquired, are simply not much interested in programming as a career(*). You can't tell them "oh yes you are interested, you have to be, because women are under-represented in this field" without denying them the right to make their own decisions about what they want to do with their lives.
It seems to me that if you want to crack this nut, you'd have to teach better parenting skills and try to reach girls at the elementary school level. By the time the woman is a young adult, her preferences are likely already largely formed.
(*) in this case, "not much interested" can be defined as "not sufficiently interested to spend the thousands of solitary hours necessary to become really good at it"