Maybe my comment was unclear due to the hyperbole. I am agreeing with you, in that one possible reason is that CS really is that bad.

But I also don't agree with your hyperbole, that 99% of CS students are subtly sexist or otherwise actively against women's participation. Maybe I am living in a bubble or something, but my experience is that CS people are actually somewhat less sexist than most of society, and I can think of other university departments (business or law) that have far worse behavior by men.

I think the geek tribe fails horribly when it comes to other things. Perhaps these are what is more important.
Understanding the emotional context of communication is one. Simple grooming and attention to social norms is another. Lastly CS students are usually quite timid in one way or another, and prolong adolescent pursuits and attitudes essentially perpetually.

I am not a straight female, and I can't claim to know what they are thinking. But I think, from their behavior, that they are repelled from CS in part because in many respects it's a low status job for low status, somewhat troglodytic men.

We are not talking about startup school here -- I mean the vast majority of CS grads, who are destined to grind out buggy Java libraries in a caffeinated haze for the rest of their days, wearing increasingly threadbare vendor t-shirts that cover less and less of their belly. Unlike law or commerce, which might be even more sexist, but at least they're around people that have the tact to hide this, and the men can communicate emotionally, respect social norms, and generally have higher status.

This might be very politically incorrect to state, but in my experience, women will be incensed and mortified at sexist behaviour by low-status males, but will rationalize it when it comes from equal or higher-status males (maybe I deserved it, maybe it's just a joke and I should forget it, etc. etc.) I mean, otherwise, why would there have been a need for a feminist movement, if not to overcome that tendency? So I'm suggesting that the sexism in CS is similar to other fields, but that women notice it more, because most geeks are incapable of projecting that power aura. And furthermore, because the field seems so low status, women have far less incentive to ignore the sexism.

Men do exactly the same thing when faced with power imbalances, if you think about it. People laugh with their boss even if he's a domineering asshole, but they wouldn't tolerate that from someone they deemed a peer or underling.

I don't want to reduce women to simple breeding machines looking for alpha males -- the evolutionary psychology approach. Furthermore, the fact that there is any sexism at all in any field is deplorable, and we shouldn't look to other fields to set our own norms.

But I'm just trying to describe what I see... in part because I feel that way too. There are many times when I look around the people I see at conferences or whatnot and feel a certain creeping horror. And wonder, out of the infinite things I could be doing with my life, why I am talking with this guy with a monotone voice and a severe dandruff problem about CSS bugs in IE6. How much worse could it be for a straight female?

This is actually why I didn't go into engineering myself (I got a degree in an arts subject and only returned to programming much later). So, maybe I'm projecting, but that's how I feel about it.