"I've previously indicated what I think Marx means by a "social being." It's an actualized species-being."
I'm familiar with the passage you quote, and thank you for quoting it, it's certainly a touchstone for a discussion about Marxism and sexuality.
But, I would not interpret it as evidence that Marxism is homophobic. For me the passage evokes the notion that might does not make right. In other words, while man might gain mastery over woman through the exercise of physical power and violence, and while he might gain a similar mastery over nature, a mastery so constructed does not constitute his humanity -- on the contrary.
As a classical scholar and as a philosopher, Marx would be very familiar with the classical distinction between productive work and reproductive work (see Phaedrus), but here is his particular genius and gift: that he did not accept this distinction. So, I am arguing that his taking up the man/woman figure as significant does not denote his homophobia nor that of marxism in general, it denotes instead his interest in breaking up, in questioning this classic and false distinction.
Joanna