Granted.

> Of course humans are indisputably sexually dimorphic

Only in a trivial/technical sense, i.e., there have to be differences for there to be distinguishable sexes. But for a biologist, humans as a species are not known for and do not stand out for their sexual dimorphism, compared to other species. Which leads me to conjecture that the differences and problems with sexism are more related to the social construct of gender.