The overt harassment - which is obviously a minority-is unfortunately what stands out in people's memories, and it only takes one person to really push someone away, and in extreme cases make someone unsafe. In some cases where I've been primarily treated like 'just one of the guys' you get one or two people in the group that don't treat me like that and regularly make inappropriate comments, but no one other than me protests that or cares. It just takes one to make you uncomfortable, and I think it's important for men to make it clear that kind of behaviour is not at all acceptable. I think Melbourne Free Software has been overall good with treatment of women in my experience, but being more aware of  your own and others actions and making sure you point out if someone else is doing something they shouldn't will always help.