First of all, because it is not the "social/discursive construction" of X in itself that makes X oppressive & a problem for feminists, Marxists, and other leftists. For instance, some languages have more words to name different kinds & degrees of rain & snow than others (for instance, compare Japanese with English), and to describe this kind of difference, "social/discursive construction" is adequate, in that having more finely differentiated names for rain & snow doesn't make the Japanese language better or more powerful than English, and vice versa, so oppression didn't & doesn't play a role in the particular "social/discursive construction" in question here. On the other hand, all aspects of gender have been shaped & colored by oppression, not just "discursively/socially constructed." This distinction is important. In other words, there are "social/discursive consturctions" (that involve oppression), and there are "social/discursive constructions" (that do not involve it), and it is the former that becomes a political question. Therefore, I'd rather emphasize how oppression (and to a lesser extent, resistance to it) created and has continued to reshape gender.
Also, to say gender is a "social/discursive construction" often misleadingly implies a degree of voluntarism & autonomy from the biological & the historical that doesn't exist in fact. Most importantly, there is no denying that the history of class & gender oppressions has been in part the history of the exploitation of the temporary reproductive capacity of one half of humanity, no? One can't simply relegate the biological capacity for reproduction to the realm of the symbolic alone, however indispensable symbolic struggles may be. Feminists & Marxists must struggle for and achieve _women's total control over our own bodies_, especially our temporary reproductive capacity, in _politics_, not just in _theory_; failing that, no amount of seemingly "refined" scholarly writings on the "discursive/social construction" of sex/gender will prevent these signs from becoming the tools of oppression.
Yoshie