(And the always popular implication that people who aren't engineers don't matter...)
Woah woah woah - all those she-beasts online had led me to believe otherwise. Hell just take a look at /r/tumblrinaction.
[0] Both many prominent Twitter Feminists and Tumblr SJWs cling to the idea that if you're privileged, you can't criticize the oppressed (even if you're not directly oppressing the oppressed in question), so if you're misunderstood as doing something that opresses the oppressed they can start a campaign against you so you lose your job, etc, but you can't answer in the same tone. This is to say, I'm being a jerk for using the word "unhinged" when I don't have any apparent mental disorders.
Some people are Assholes. Feminists are people. Ergo, some Feminists are Assholes. It doesn't say anything more about feminism than it does people in the checkout line at the grocery store.
Here's another feminist who also did the same (and was eventually made to resign for it, thankfully): http://en.wikipedia.org/wiki/Mary_Daly
The vast majority of tech industry feminists are postmodernists.
There's a lot more to this phenomenon than you could get into on hacker news, but it goes something like "if you don't agree with everything i say exactly, you're clearly the enemy"
The 20% number definitely is not from the tech industry either.
So you have women picking different jobs. You have different incentives and preferences (motherhood is the big one here), different preferred industry (like media vs mining or whatever).
You also have women getting half of the income of their husbands, having the option to drop out of unpleasant jobs, not being under the same pressure to feed their family, and so on.
There are many many aspects to this. So far I haven't seen anything that convinces me women are being treated unfairly.
There are issues, but only if you consider staying at home with kids degrading. For example I suppose the incentive to get a good education is less if you know you will miss out several years where you could earn back the money invested. I don't think "unfair" is the right way to characterize that issue, though.
There is also the popular implication when there is a girl on twitter starting trouble again, they're not coding and trying to claim some form of discrimination, instead of developing programming skill.
You can't--and obviously I mean in the "if you're a basically functioning, mostly vertebrate human being it's morally repugnant" sense of "can't", not the physical, over-literal sense that I feel like you're going to use--blame shitty workplace behaviors (and this isn't the first I've heard of GitHub having problems) on people not knowing how to code. I mean, what kind of world do you want to live in where you can excuse mistreating people because they don't "develop programming skill"?
Actually, don't answer that. I don't want to know what kind of world you want to live in.
Edit: I see from your profile that you're an undergrad. Hopefully you'll grow out of the attitude that only super-coders are valuable to a project, like I did.