In other words, "binary gender"gender"* has an individual as a target, not the society/population on the whole. When they write "the idea of male/female," they mean "the idea that one person can be g1:male and at the same time be g2:female." Well, most certainly is it true that that "is notnot generally considered a normnorm of contemporary Western civilization." Yet.
Q.E.D.
*A binary gender is a gender that is neither male nor female. It is a third thing. Nevertheless, it is the binarity that makes it the third.