I expect you've only noticed one case of sexism. That only one case was overt enough that, despite being a man, it affected you. Many women in the field have countless stories of discrimination. The article I think you're referring to was a parent complaining about a teacher not reacting to harassment and bullying her daughter was receiving simply for being female in a computer science class. To the extent that her daughter no longer wanted to study computer science. That's a far cry from arguing in favor of "positive discrimination".

But here's the thing: it doesn't really matter what you have or haven't experienced. What matters are the experiences of the people being discriminated against. As I said, these stories are everywhere; if someone says they were made to feel a certain way (such as: unwelcome) it's not your prerogative to question whether that was a valid way to feel.