For years, I was the one&only technical woman in the different companies I worked for. That can be very difficult, especially in a start-up environment where (depending on the culture), the atmosphere can be as sexually charged as a college dorm. I've had supervisors hit on me & grope me at parties. Another boss told me he was going to replace me with a nice, docile set of Geisha programmers, or sell me into white slavery if the start-up folded. The new sales guy at one place asked me to get him coffee, and when I explained that wasn't my job, he said I was "too hot to be a programmer." I was not flattered.
And then there are the message boards where hoards of young men post their opinions about women being intrinsically incapable of hacking. You have to have a really, really thick skin to be a woman in this line of work.