It's been a while since I cracked open my Lacquer (the scholar that's most associated with the idea of a shift from a one-sex to a two-sex model of male/female difference).

But from what I can recall, Lacquer's basic argument was that the one-sex model became too risky in that allowed for too much blurring in the lines between male bodies, traits, and privileges, and those of women. If men and women were just variations on a single physical model, it could still be maintained that women were an inferior manifestation, but the idea still left room for variations along the spectrum of that single model and allowed the possibility that some traits and privileges might become unisex or also exist along a spectrum.

The two-sex model prevented this by re-imagining men and women not as different manifestations of the same basic body, but as fundamentally different models whose traits, spheres, privileges, etc did not and could not overlap or lie along a continuum.