That's right, some fish change sex (ie stop producing either small or large gametes, and start producing the opposite gametes) and this is simultaneously proof that sex doesn't exist at all, that it exists but has nothing to do with gamete production, that it exists but on a spectrum defined by variation in primary and secondary sex characteristics, that it exists but is defined by brain structure not reproductive roles, that it is immutable in the sense that TW have always been women and this can never be changed, and also that it is mutable in the sense that anyone can change sex by having surgery and taking hormones. It couldn't be simpler, you all just don't understand biology.
In other news, some lizards change their skin colour so this also means Rachel dolziel is literally a black women; non human animals exist so all furries are literally the animals they dress up as; caterpillars grow into butterflies so I can also grow wings and fly if I want to etc etc. Anything that exists in nature, even if it only exists in non mammalian species, is evidence that the same thing can and does occur in humans. That's just good science and anyone who says otherwise is a bigot.