Last year at FooCamp I went to a panel about women in tech (I believe I was the only guy) for two reasons:
1) I have no qualms about offending people if I feel my point is valid, and in fact enjoy being in situations where I risk such things (I find it incredibly rewarding to have meaningful discussions about things that are considered taboo).
2) To find out what the problem was
Let me clarify #2: I didn't see a problem within my circles other than the fact that there were few women in startups. I didn't see blatant sexism, I didn't see harassment, I didn't see something that I was comfortable changing to encourage women in my industry. I still occasionally quote South Park or make a dirty joke, but nothing targets women specifically: I just can have a crude sense of humor. Was that what women were concerned about? Were they offended by my jokes, expecting me to be stiff as a board to avoid offending them? In my opinion that's expecting special treatment and is sexism in and of itself.
In my YC class the 4 women were respected and treated like peers. Yes, I'm sure there was some flirting, but I didn't see anything that I wouldn't consider more offensive than I've seen guys do to each other on a regular basis.
What I learned was actually very interesting: from their perspective I was not the problem. Somehow in the middle of the media hype about sexism, I had assumed I must be doing something wrong and was trying to figure out what was inappropriate about my behavior. It turns out there's a whole group of men in tech who are blatantly offensive, harassing, and generally sexist towards women just because they were women. I genuinely think most men are in the better bucket: we aren't offensive, and provide mostly non-hostile work environments. We are not part of the problem. On the other hand, I bet a lot of us aren't part of the solution.
This needs to not be a "men vs women" battle: this needs to be a "people vs jerks" battle. There are a few sour apples poisoning the environment for everyone, and it needs to stop. I do think the only way to fix this is for there to be a social change, but most men just need to change the way they act towards other men. Don't allow it to be ok when one guy crosses the line, and don't just assume HR will take care of it. Call him out on it. I'm confident that that's all 99% of guys need to (and can) do to help with this problem.