A point I'd like to raise, based on my experience of engineering, is that even within engineering, females tend to gravitate into certain areas... or, basically, away from anything that is big and clunky. At the companies I've been at, there has been a much higher proportion of females in areas like materials, process, etc. as opposed to, say, mechanical.
I also think it can be quite daunting for a girl to go into engineering, especially if she wasn't the type that played with Lego or Meccano. Not because she'll face sexual discrimination, but because she'll likely be amongst people who did play with Lego and Mecanno from a young age, who have essentially been doing engineering from a young age, and they'll be talking a practically different language.