Even if you want to separate sex and gender and define sex using sexual characteristics (not actually a good idea, see works by Judith Butler and Julia Serano among others, although I wont fight that point here), almost no sexual characteristics are immutable. The only ones that I can think of are chromosomes and gametes, but chromosomes aren’t even binary (or observable without a microscope) and gametes arent a good basis either – should being infertile affect your sex?
Even if you want to separate sex and gender and define sex using sexual characteristics (not actually a good idea, see works by Judith Butler and Julia Serano among others, although I wont fight that point here), almost no sexual characteristics are immutable. The only ones that I can think of are chromosomes and gametes, but chromosomes aren’t even binary (or observable without a microscope) and gametes arent a good basis either – should being infertile affect your sex?