In _The German Ideology_ , Fred and Karl do recognize reproduction as a "fundamental condition of history" along with production. However, they give reproduction, or at least, "the family" a subordinate "fundamental" status to production when they say:
"The third circumstance which from the very outset, enters into historical development, is that men, who daily remake their own life begin to make other men, to propagate their kind: the relation between man and woman' parents and children, the family. The family, which to begin with is the only social relationship, becomes later, when increased needs create a new social relations and the increased population new needs, a subordinate one..."
My thesis in this comradely critique ( I really do love Big Daddy Karl and Uncle Fred overall) is that the mode of reproduction (in the broad sense, including, but not limited to social institutions called "the" family) of human beings remains throughout human history ,even after classes arise, equally fundamental with the mode of production in shaping society, even with the "new social relations" that come with "increased population." For there to be history in the sense of many generations of men and women all of the way up to Marx, Engels and us today, men had to do more than "begin to make other men." Women and men had to complete making next generations by sexually uniting and rearing them for thousands of years. Otherwise history would have ended long ago. We would be an extinct species. An essential characteristic of history is its existence in the "medium" of multiple generations. Thus, with respect to historical materialism, reproduction is as necessary as production. The upshot is women's liberation must be put on the same footing with workers' liberation in the Marxist project.