Are we (developers) the douchey 80s guy of the 21st century?
To paraphrase Neal Stephenson's Snow Crash, one problem is guys believing that they're too smart to be sexist.
I also believe that a lot of people involved can't handle the idea that they're being bullies, and so further attempt to rationalise their actions with terrible analogies and arguments that boil down to "I'm a geek - I'm incapable of oppressing someone!"
I think to a large degree it's also about not understanding group dynamics. Let's say that:
35% or so of geeks are closet sexists in reform: aware that their belief systems are wrong / socially unacceptable and trying to change.
5% or so aren't sexists but misogynists: men who truly feel wronged by women and wish to retaliate.
60% are perfectly stand-up men who wouldn't do a bit of harm
Now the trick is that whenever the sexist and misogynists form a group where they have a significant majority, the closet sexists will give the misogynists a platform to damage women.
Meanwhile the other 60% sees nothing but 95% of perfectly respectful men who would do no harm against women and can't even imagine something like that happening in their company.
This isn't a problem with guys or with developers in general.
The vast majority of people who participate in conference are good people. However, it only takes one person (out of thousands!) who has this kind of personality dysfunction to lead an attack, and women are much much more likely to be the target of their dysfunction.
To paraphrase Neal Stephenson's Snow Crash, one problem is guys believing that they're too smart to be sexist.
I also believe that a lot of people involved can't handle the idea that they're being bullies, and so further attempt to rationalise their actions with terrible analogies and arguments that boil down to "I'm a geek - I'm incapable of oppressing someone!"