OK, so the answer is obvious - the biology of our bodies. It is what differentiates women and men.
But I've been thinking about this some more - there is NOTHING that I, as a woman, have in common with all other women, except biology. We may, as a group, be lower paid than men, or more likely to wear lipstick, or to empathise, or to do the school run, or be sexually assaulted. There are also things that some women do that no man does, like give birth. But not all women do or feel these things (and some typical traits or experiences are shared by some men), so they cannot be defining characteristics.
The more I think about it, the less I understand trans people. How can you identify as a member of another group, when that group is large and amorphous, and when that group's ONLY defining characteristic is something you can never share.
Maybe I am asking in the wrong place, as I expect most people on here agree with me. Can anyone explain?