> Nurse sees a penis on the baby - writes M on the birth certificate

Sex is determined at conception and parents today often know what it is before birth.

> Non binary people think those gender roles don't make sense for themselves, and in that respect, I think almost everyone is at least a little non binary.

You only say that because of rigid thinking about what boys and girls are.

When someone says to a boy "stop crying like a girl" he's displaying a rigid definition of what a boy is. Instead of saying that some boys can be very sentimental too, you're saying he's non-base-2. That's no different from the person saying boys shouldn't cry.