I've seen this claim before: that if you don't think sex can magically change, you don't regard a trans person as human. And it's just so stupid.
It's irrelevant whether you're a man or a woman. You're still human. We all are. The bizarrely overwrought attempt at moral coercion, in pretending that anyone pointing out sex as an immutable reality is thereby denying anyone else's humanity... it's bonkers, and it's dishonest.
Of course trans people are human. They're just not the opposite sex to their own, and never can or will be, though of course they should be able to express their gender in whatever way feels right to them. What part of that is hard to understand?