I just read through this thread - it seems to me that people are talking at cross-purposes here. Would anyone mind if I submitted some banal points, to try and clear things up a bit?
1) The term "Black" (like the term "White") clearly has no kind of relationship at all with biology. It is socially constructed, discursive, and scientifically meaningless. But it does have a socio-historical meaning, which it is possible to think about.
2) The question of gender, plainly, is not quite as simple - if only because it invariably tends to slide into a question of biological sex. There are, it is clear, male and female bodies, even if the borders between them are sometimes fuzzy. Sexual preference is a yet trickier matter, and clearly has something to do with language and the desire inherent in it.
3) It does not follow from the "social constructivist" position - and the idea that reality is in some sense constructed by language - that everything in reality has equal reality. Just because both reality and race are socially constructed, race does not thereby become real. Though all men are mortal, and Socrates is mortal, not all mortals are Socrates. As this thread has demonstrated.
4) Social constructivism is basically a form of sociological nominalism. It adopts a stance on universals, and suggests that these are historically and discursively generated. It then attempts to show how this happens. Social constructivism is not hostile to science tout court, despite what Sokal and Bricmont seem to believe: there is no way in which a commitment to social constructivist thinking would force someone to grant that "race" is as real an entity as, say, atoms, despite the fact that it recognizes that both are in some sense socially generated. Social constructivism does not equal sheer relativism, and to claim that it does is to construct a strawman.