> I don't want my daughters sucked into dangerous oil field work prone to boom and bust cycles.
Are you implying that it is ok for your sons to be in such a field?
To answer your question though...
To establish a baseline, women get paid, as a whole, less than men for a particular job title (women doctor paid less than male doctor, women electrician paid less than male electrician). This is on a statistical basis when normalized for experience, location, etc. It may not be that eg. a particular iTunes team at apple has this imbalance, but across all industries, across all companies, across all locations, this statistic emerges. This is wrong, because men and women are equally deserving of money. Thats not politics, that's human decency. To quote an SNL skit "she made the unfortunate choice of career after getting a law degree. Instead of working a lawyer, she chose to work as a female lawyer".
Beyond this, women also tend to occupy jobs that pay less than men (teacher vs lawyer, nurse vs SWE, etc). This means, again, that as a whole, women in society have less money than men. People tend to be skeptical in the issue here because of the perception of choice a women has over her career. I (a man) have heard from countless women in engineering school and later as a professional that women are made to feel like they don't belong. My own mother (who has no reason to lie to me), an engineer, made the newspaper for being one of the first females licensed in her field. She told me countless stories of men passively trying to make her feel like she didn't belong. This sense of not-belonging means that the "choice" to work a lower-paid job is less of a truly independent choice and more of a societal nudge.
Now that there is some established evidence of why women make less than men, I'll let the reader decide if its wrong.
Now that we've decided it is wrong that women are paid less without choice, we should try to fix it.
How should we fix it? Lets get women into higher-paid job that's don't have many women! But which ones? Like you said, you don't want your daughters in oil fields, but surely you'd be much happier with your daughter in an office building somewhere typing at a computer? Maybe something with good WLB? Maybe without the need for a decade of education (and loans!)?
Great! SWE is a job that pays well, is safe, and has a generally "easy" way to enter - just a bachelors degree, and amble internships. Bonus is that all the equipment you need to learn is a computer (which most people have), so that's a lot easier to supply to poor kids who may have more trouble getting lessons.
Other jobs that are high-focus for female-occupation are doctors/lawyers/finance. Those jobs also pay well but have higher education requirements, or worse WLB, or are harder to get. But some (like doctors) have huge prestige. Who doesn't want their daughter to have a safe, high-prestige, well-paid job? No one is trying to get their daughter (or son!) to work on a crabbing boat - one of the most dangerous jobs in America!
TLDR: Why do we as a society focus on ending the professional gender ratio mis-alignment on certain jobs? Because no one wants those other jobs (for any gender really), and the goal is to get women equal opportunity for salary or equal prestige, and that is easier in some fields.