Well, I got bullied out of my Physics A-level at a mixed school by the male teacher who didn't think girls should be taking physics. I was also the only girl doing CDT at GCSE - purely because previously I'd been at a girls school, and had to pick my options without seeing the school, and didn't realise that it wasn't the done thing for females to do CDT.
On my degree course I was one of never more than 4 women in lectures of up to 100. In my jobs I've always been one of one or two women in my department (IT). When I go to conferences I'm in a distinct minority, and often the only female speaker.
Growing up, I often gave in on getting a turn on the computer because the boys at youth club, or my male cousins at family houses were reluctant to take turns, and very aggressive when I finally did get a turn.
I've seen women be managed out (pushed into project management roles), voluntarily leave (from helpdesk where she was getting continuous harassment she just gave in and went to work in HR).
So I think it's a combination of not seeing women there, and just a continuously slightly hostile environment - one where you're always the outsider, where you're always a problem because you can't share a room on work trips, because your (obviously male) boss can't take you on the sales or install trip because he's worried how it will look, because socialising outside of work with your colleagues means being the only woman in a group of blokes, and that can go south fast.