But those who claim that sex is determined by chromosomes (or secondary sex characteristics or hormone levels or gonads) must not realize that sex is assigned at birth not by chromosomes, but by genitals. When you are born, the doctors do not look up what your chromosomes are, they look at your genitals and determine it based off your genitals. The vast majority of us never learn what our chromosomes are, we just assume it based on being told that we are either male or female. A person can have a penis and not have XY chromosomes and live their life under the notion that they have XY chromosomes simply because they have a penis  and they could be wrong.
Genitals can be ambiguous at birth that do not meet our strict perception of what our genitals should look like (as in being normal), which is inconsistent with the gender binary. Our genitals are not unchangeable, which is also inconsistent with the gender binary. I think one of the most important things to recognize here is that if a trans woman doesnt have a penis, most men still claim that she is still a man  which is inconsistent with the entire social construct of Biological Sex. Under this notion, if man is a synonym for penis, but a person does not have a penis, they cannot be a man. So how could a trans woman who does not have a penis possibly be a man while not having a penis, the definer of a man?