From the ‘Private Property and Communism’ in the Economic and Political Manuscripts (Milligan, trans., New York: International Publishers, 1964) :
The direct, natural, and necessary relation of person to person is the relation of man to woman. In this natural species relations man’s elation to nature is immediately his relation to man, just as his relation to man is immediately his relation to nature–his own natural destination. In this relationship, therefore, is sensuously manifested, reduced to an observable fact, the extent to which the human essence has become nature to man, or to which nature to him has become the human essence of man. From this relationship one can therefore judge man’s whole level of development (134).
Leaving aside the persistence of Hegelianism (or Feuerbachianism, if you prefer), there’s a lot to say.
On the one hand, we have the recognition that gender-based exploitation is a problem, and a prototype for pretty much any other form. He will repeat this in The German Ideology. There are surely no Marxist grounds for ignoring, minimizing, or treating as secondary and derivative the question of the status of women.
On the other hand, and especially if we treat “the relation of man to woman” as a cipher for sexuality, which is precisely what Marx has been discussing above (“general prostitution” (133), “the approach to woman as the spoil and handmaid of communal lust” (134)), we have one of those moments when it’s difficult not to feel as if Foucault was right about everything. Can we not say that sexuality appears as the secret essence, the obscure principle which gives intelligibility to the rest?
Remarkably, however, this is precisely the reverse of what Marx is saying. Sexuality is not at all hidden. It is, on the contrary, the sensuous, visible element that allows something else to appear: species being, the essence.
This leads to more questions. First, is what is at stake here really sexuality in the sense of Focuault’s dispositif? Marx’s focus on visible acts suggests that it may not be, at least if we take the ‘secret essence’ to be essential to sexuality in Foucault’s sense. Second, if sex makes visible a set of relations that are otherwise more difficult to see, can we say that sexuality, with its postulate of an obscure essence, complicates this signal value of sex, making it more likely to function as a screen?