The majority of cells in a woman's body have what's called a Barr body. You can look under a regular light microscope and see if it's a woman's tissue or a man's with ease. That there are 0.0x% of the population with something besides XX or XY doesn't mean the binary is wrong, it means there's oddities at the fringes. In much the same line, a woman with a mastectomy is not less a woman than a man with bolt-ons.