I don't personally believe this discrimination exists on the scale that it is touted. I think it starts much earlier. As far back as college, women were more interested in Nursing, Acting, Business, English, Philosophy, etc., but rarely would I sit at the advisement panel and hear "Well, I really do like computers!" from a young woman.
That's not every woman, but a very big chunk that I witnessed.
Our school began participating with various groups to secure funds to help encourage women to come into the CS, MIS, or SE programs. I can count on my hands how many additional women joined. Nowhere near enough to call the effort a success. More women chose the classical major choices instead of the various computer fields, even if they didn't show a lowered cost of attendance. Why?
Through it all, I'd have to blame the culture. You don't fix the problem by throwing money at it. You fix it by showing that Penny is just as comfortable around computers as Sheldon is to Theoretical Physics. A field doesn't have to be stylish, and a playing with a computer doesn't have to make you a nerd.
I assure you there's no concerted force working to eschew women from the sciences. If the culture fits and is likened by its followers, I'm sure the numbers will grow to equal measure of the men. If you start from the root, you can fix many of the problems we see today.