The silent woman in class

Today in class someone said something (I can't remember what exactly) that was pro gender equality and someone else in my class (someone who I have difficulty with normally) said something anti women in response, which he found uproariously funny. I did not. I bit my lip as my fists grew tighter and tighter and my thoughts became more chaotic. When class was over I walked out as fast as I could without running and didn't look back. I couldn't walk fast enough on the way home, the rubber soles of my shoes slapping against the concrete, icy air numbing my face and ringing in my ears. When I got home I cried. I don't think it was really that that person intentionally said something sexist (although it was so , particularly in a classroom setting, particularly in a class that has discussed the issues of racism (amongst other things) for the last year, and particularly as it was already a male dominated environment) that upset me, but the fact that I kept quiet. I don't know why I go silent, stormy and eventually teary when I am offended, hurt, or don't agree. Why can't I stand up for myself? With the upcoming presentations on feminism I am particularly worried about how I am going to handle, will I be able to say anything at all?