Gender is mutable. Sex is determined by genetics, though in a handful of creatures, sex could be considered mutable.

You're right, but there was an article on here in the last week about genetic therapy correcting Down Syndrome.

I wonder how long before sex does actually become mutable.

You can change sex

And people can be born intersex

Your data is wrong

The fact that some people can be born with 6 fingers doesn't mean that saying "a human hand has 5 fingers" is generally false.

It also means you can't use the generalization "a human hand has 5 fingers" to then say that "therefore because your hands have 6 fingers, you are not a human"

I the sense of what OP did say, it would be more "grafting a sixth finger doesn't mean that you are not human anymore".

Just like taking meat from your arm to create new genitals doesn't make you a man, biologically (psychologically it may be different).