It’s true the legal definition of sexist is discriminating against someone based on a gender and it could be considered discriminating to make a choice not to be friends with someone based solely on gender but, even if you made a conscious decision not to be friends with women, I don’t consider that sexist.
Here’s why - We are not dictionary definitions. We are living, evolving humans with complex thoughts and emotions. To put us in the box of a book definition is, in my opinion, not logical nor productive.
We all have people we related to better than others. We are drawn to certain types of people and not to others. It’s one thing if you treat someone poorly because of their gender but it’s entirely another thing if you don’t relate