What a great question! Honestly I could not pin it down to one experience, but rather a series of experiences. Generally speaking, I would say a change in my outlook has always been associated with key developmental periods in my life when social and personal expectations/pressures have markedly changed. I can't go over all of them. So I will address my answer in the context of why I am on HubPages to begin with. The emerging issues facing men in society and the sexism we face.
One experience with girls during boyhood and adolescence-
I am Gen Y and first remember realising something was amiss when I was nine years old. There seemed to be an air of haughtiness among the girls at school towards the boys. Initially I thought it was just a phase, like girl germs, boy germs etc. But it was more than that, they had this mysterious disdain toward masculinity. I remember being very confused by it because it seemed to come from nowhere without provocation. At school in general there seemed to be this dislike of maleness. Boys were never praised and encouraged like the girls after they did well at their schoolwork. There was little or no opportunity to engage in male behaviour in the classroom (in sport only). When typical male behaviour was recognised it was mostly the bad kind, not the good kind. Girls were also often given special treatment. Boys in my class were often punished in ways to humiliate and yet I can never remember a misbehaving girl receiving such punishment. I can even remember special treatment on the playground. Girls could sit in areas that I was told I could not sit in. Of note was the fact there was only one male teacher at school. But it was this prevailing pro-female and anti-male attitude of both the teachers and the mothers (not just the teachers), that was hurtful to me.
Female haughtiness had mushroomed by my adolescence. If you had an innocent crush on a girl, you were a stalker or creep. It was always up to the guy to approach the girl and hope he did not get painted as some deviant. Girls seemed to revel in rejection and rumor mongering. Whilst picking up was up to the guy, dumping was pretty much the girls territory. Dating seemed to be mostly about the girl, with little concern for the guy.
Fortunately I have met some nice girls since then. But they seem to be a minority. Perhaps it is the country I live in (Australia). I don't know the exact cause, but I suspect the hostility against men has a mostly cultural basis (at least in the West).