We believe that sexual politics under patriarchy is as pervasive in Black women’s lives as are the politics of class and race. We also often find it difficult to separate race from class from sex oppression because in our lives they are most often experienced simultaneously. We know that there is such a thing as racial-sexual oppression which is neither solely racial nor solely sexual, e.g., the history of rape of Black women by white men as a weapon of political repression.
This paragraph is a great example of the importance of intersectionality. When the authors write that their is such thing as racial-sexual oppression that is not just racial or sexual, it's both, and therefore different. Though the term intersectionality did not exist yet, the idea of it is touched on here.