I think it IS sexism, but not in the way it's usually thought of. I think that this happens without anybody being overly prejudiced, but tends to happen to every minority or marginalised group. When a profession is made up almost exclusively by a given kind of person (white people, males, straight people, Christians and so on), even if they open their doors to minorities, a lot marginalised people are going to prefer to stay in the professions they were socialised to view as acceptable, and only the really brave ones will take the chance.
The problem is that when women make their way into an exclusively male profession (or any marginalised person makes their way into a profession occupied exclusively by people who are not like them), there is always a change of the status quo. If men are used to telling raunchy jokes and the woman feels uncomfortable as a consequence, she has a tough choice to make: will she just take it and be quiet, slowly growing resentful of the atmosphere? Or will she try to raise the issue with her superiors? The issue can get dismissed under a "boys will be boys" rationale, or actually get taken care of, and thereby cause the men to resent her for forcing them to change something they had got used to. And even if she doesn't complain or raise the issue, the men can end up changing that themselves to avoid making her uncomfortable, but they can't avoid the inevitable resentment that will eventually come to happen as they have to change their ways to accommodate for the newcomer.
The same thing also happens to LGBT people in a straight environment. When people say "Ugh, that's so gay!" or make a gay joke, the LGBT person is in the same position as the woman in the previous example. What do they do? Do they just put up with it or risk everyone resenting them? The same goes for a person of colour on a profession full of white people.
At the end of the day, people aren't likely to brave an environment that feels unwelcoming, and where every day you are reminded that you are fundamentally different from everyone else, that you don't fit in and never will, and that if you aren't careful, then everyone will turn on you.
Now, before anyone brings it up: yes, it's possible that, on an individual basis, this won't happen. It's possible that you can be a woman, LGBT person, or person of colour who finds a very tolerant environment where you fit in perfectly and there's no resentment from anybody at all. But these are exceptions. Most of the time, inserting yourself in an environment full of people who are different from you will turn out as I described above. I and many others can vouch for that from experience.
So yes, women make their own choices and choose not to enter the industry, but I personally can't blame them at all.
