I don't think most scientific girls are bothered by there being not many girls in physics or chemistry classes. I loved it, no bitchy cliques. No fashion and celeb rubbish to worry about. Geeky boys don't expect to get girl friends so they are just friends and lab partners.
I really think there is something in the very abstract nature of physics and computing that doesn't appeal to most Women. Strange as pure maths does have quite a few females.
DH is a total geek and he can get totally engrossed in fine technical detail in a way I can't.
I love the bigger picture the big bang theory, partial physics etc are fascinating until you turn them in to maths.
What computers can do is brilliant, but writing code with all those fiddly rules
I think we most certainly need to encourage girls into physics and computing, but I'm not sure we should expect equal numbers to study them at the highest levels or be found in the most abstract corners.
Perhaps it is hard wired into us to always keep on ear out for the baby crying in the night, so to speak, to not do one thing and ignore everything else around us.
I don't know, it's bloody annoying, a bit of me would still loved to have been an astronomer.