I've been thinking this out for some time. Why does it anger the TRAs so much? Each part of it challenges them.
1 Adult. For a long long time women have been regarded as infants, in some cases openly and legally. In 1976, after the change in law, Halifax asked for my father to guarantee my application for a mortgage. He saw them off. No-one would now state this idea openly, but they still think it, or at some layer of the brain feel it to be true, and they don't want to change this.
2 Human. Of course, the default for human has been, for millenia, the male. We don't have the right dangly bits, so don't make the Linnean definition. We can't be human.
3 Female. Since they now have inserted some men into this, we can't have it without being transphobe.
So adult human female can't be true, and they viscerally reject it. By their man brains.