This is the physical matter of fact. This fact does not carry with it any sort of dictation as to the role we should play in society, how we should dress, or what job we should have.

Natural selection favored gendered behavior long before the first hominid hominated. What is there about homo sapiens that makes it able to eschew gender as a survival strategy? Why would our genes not push us to behave in a gendered way, given that every critter we are descended from is genetically programmed for gender specialization? Is that feature magically missing from our make-up? And if so, can we simply state that's the case because we want it to be the case? Where's the science? We can't find the gender gene in the other animals, but they have gender. Why are we any different?

Genes aside, think of the engineering problem. In a phrase, mama's baby, papa's maybe. How can our societies not evolve mores that assure a man that his kids, the ones he risks his life to feed, are his own? Gender roles see to that, as best it can be seen to, with varying degrees of success. The strict Muslim rules about gender seem designed entirely to assure a man that his children are his own. In a harsh environment, that goal may have greater urgency than in a land of milk and honey.

It's really quite easy in infer that gender will arise from the physiological nature of our species. Blinding oneself to the obvious takes a bit more work. But, obviously, it can be done.