Sex stereotypes are not arbitrary, but they're also antiquated and unnecessary in a modern society. I believe we should get rid of the sex stereotypes which are commonly called gender, but that doesn't mean that sex doesn't exist and have a significant meaning.
The hard line between men and women exists. Unless you're counting intersex conditions and genetic anomalies, only XX and XY individuals exist.

You can be gender non-conforming and there's nothing wrong with that, but it doesn't change your sex. Pretending that sex doesn't exist would erase all of our sex-based protections among other things.