Gendered words aren't sexist (he/she), they are descriptive in the same way that black/white are.
Bitching is the (sort of) equivalent of the word niggardly. Niggardly has no etymological roots with nigger, but they sound similar and niggardly means stingy/miserly/has negative connotations. Lets pretend for a second that they are derived from the same root (They're not but I lack a better example off the top of my head).
Saying black would not be the same as saying niggardly.
