Gender is masculinity and femininity. They aren’t real, and are imposed by patriarchy to keep the binary around.
Sex is real, biological, and fact. Males and females. They’re defined by reproductive systems, and science. Obviously an individual from either sex has a reproductive issue, they’re still that sex - because what they have is an abnormality rather than proof that sex doesn’t exist. Intersex is a medical condition and not proof of another sex.
Eradicating gender does not mean eradicating fact, like sex. It means eradicating something that has been falsely imposed to dictate power imbalance. Women are not necessarily feminine, as men are not necessarily masculine. That’s 101 feminism. Anyone who supports gender supports the idea that there is innate femininity with females (and masc w males). That perpetuates patriarchal power imbalances between the sexes. Gender is an oppressive tool. Stopping the idea that if you’re ‘feminine’ you must be a ‘woman’ and if you’re ‘masculine’ you must be a ‘male’ would be a start, in abolishing it.