I’m aware there’s a distinction between gender identity (psychology), gender expression (sociology) and sex (biology). I used the "biological gender" term since the comment I’m answering to did. I could have expressed myself clearer though. I’m just gonna use the fact English isn’t my mother tongue as an excuse here…
Generally my argument is „sex isn‘t binary (and shouldn’t be regarded as such), so why would gender (both identity and expression) be?“
Besides, sex is a spectrum beyond chromosomes. Even two XX or two XY people can have differing physiologies when it comes to hormones, etc. resulting in one appearing more masculine and one more feminine.