I think this is just about power. Where do women, as a class, differ from men functionally, if we don't differ in material ways like having a uterus or ovaries? Well, on average we're smaller, weaker, slower, less physically robust, need more frequent contact with medical providers, need more societal protection of our physicality. If you redefine the class of women to include people who are actually male, you distort the statistical curve of any feature that affects our class. In fact you erase the physical differences that vary on a continuum. Thus we have males able to defeat (with ease!) females in running, cycling, etc, and the rationalization: "some women are really tall/really strong/really fast, so what's the big deal?" as if females who have similar features as males are not extremely rare in our sex class. What is the overall societal effect of this? It takes away protection and resources from those of us who, because of our biological femaleness, are smaller, weaker, slower, and erases the evidence that we even need that protection. It solidifies power in the hands of the powerful.

Everything about this is really about power.