The 1949 book The Second Sex is considered one of the landmark books in feminist literature, and Simone de Beauvoir's magnum opus.
While de Beauvoir's central thesis is that women have always and are still considered a "second sex" in relationship to men, and second-class citizens regardless, she also makes the distinction between women and other historically discriminated groups. For example, she says:
...no group ever sets itself up as the One without at once setting up the Other over against itself... Jews are ‘different’ for the anti-Semite, Negroes are ‘inferior’ for American racists, aborigines are ‘natives’ for colonists, proletarians are the ‘lower class’ for the privileged.
Her distinction is laid out specifically in a later paragraph:
...proletarians have not always existed, whereas there have always been women. They are women in virtue of their anatomy and physiology. Throughout history they have always been subordinated to men, and hence their dependency is not the result of a historical event or a social change – it was not something that occurred.
Simone de Beauvoir's comparison, therefore, is that while women and the proletariat have similar discrimination and prejudice, the proletariat are a creation of the bourgeoisie -- the lower-class as a whole are a creation of the upper-class -- while women are a creation of the male superiority complex and of history. Since women have always been lower-class, the "One" is male, while the "Other" is female -- even the words female and woman are subordinated to the root words male and man. Therefore, the root problem lies in the historical and cultural distinction between the sexes more than in the deliberate placement of women as second-class.