The argument is that the availability of choices is not the same between the sexes. Part of the problem is that so few women are in the engineering disciplines that when young girls make the decision (and this is of course, a simplification...it's not the decision, but hundreds of little decisions that factor into a career choice), they aren't as inclined to go into engineering because they just don't see themselves as being engineers. Nor do their parents, peers, social groups, etc. So it's an unfortunate feedback loop.

To use an anecdotal example...I wanted to be a journalist growing up, but that was not something my parents heavily encouraged, when all of the other Asian immigrant kids went into med school or engineering. I ended up just doing journalism and engineering in college but I credit that to my parents being relatively easy-going about what path I choose. I'd like to say that I graduated with an engineering degree because I'm especially talented for it, but more likely, it's just that I didn't face much resistance and I also saw a lot of other people "like me". I doubt this is the same for women.