Theirs is not just an opinion when you have a few decades of sociological research to back it up. If you want to see research supporting the claim that you are wrong a good starting point would be Project Implicit's webpage.

Indeed, the person that responded to you could have been less inflammatory, but it is pretty hard to keep it cool when you are surrounded by people that dismiss an entire group of people based on gender, sexual orientation or skin color.

FYI, the definition of sexism I'm using is the dictionary one, not "things I disagree with". I called your opinion sexist because it is sexist. Here's the definition:

     1. prejudice or discrimination based on sex; especially discrimination against women.
     2. behavior, conditions, or attitudes that foster stereotypes of social roles based on sex.

If you want to argue, instead of going all ad hominem and somehow trying to make it be about my supported ignorance or inability to handle disagreement, let's actually stick to the issues.