Gender-based prejudice is a separate thing from sexism (but they are interrelated). Point one and three from this page [1] do a good job explaining, but here's the crux of the argument:
"The first important thing to understand about sexism is that it is systemic. Gender-based prejudice, on the other hand, is individual, although it exists in the context of a sexist system."
"But men who experience oppression based on the way they express their gender are still not oppressed on the basis of their gender itself. That is to say, they’re not experiencing that oppression because they’re men. Non-men, on the other hand, experience gender-based oppression because they aren’t men."
"The first important thing to understand about sexism is that it is systemic. Gender-based prejudice, on the other hand, is individual, although it exists in the context of a sexist system."
"But men who experience oppression based on the way they express their gender are still not oppressed on the basis of their gender itself. That is to say, they’re not experiencing that oppression because they’re men. Non-men, on the other hand, experience gender-based oppression because they aren’t men."
[1]: http://everydayfeminism.com/2015/01/sexism-vs-prejudice/