"They seem to be muddling up the fact that primary and secondary sexual characteristics vary in appearance (breasts range from cup size AA to K, penises from micro to porno, some men can barely manage a goatee let alone a full Captain Birdseye Beard) with sex itself (two types of gonads, two types of gametes, two types of genitals and an entire body arranged around that reproductive system, eg women having more body fat and a wider pelvis).
I blame Mermaids and the GenderBread Man and that sodding unicorn."
I am trying to figure out what is at the root of it and I'm not convinced we have yet worked out the cause. Scientifically, I think the error is in thinking that all these disparate effects of sex (breast size, interior reproductive organs, height, shoulder span, etc) can be somehow summed onto one axis. In that model, ok, you can use estrogen to cause breast to grow, or you can use mastectomy to remove breasts, and can thus create the appearance of the desired sex. In reality, sex is along one of two pathways (male or female) that affects you throughout development. Some features are not affected by the pathway (hair color probably a good example). Some individual downstream features can be modified at one particular time, whether by surgery or hormones. Developmental disorders, illness, and perhaps some environmental factors, can affect the pathway at various point in development. But the pathway itself will continue to affect each cell of the body in a way that is governed by the person's sex.