Simply put, no one is denying that biological sex is a thing. You're still biologically "male" or "female" even if you're trans (which is why the term "trans" is even a thing!), or to be more accurate, XY or XX.
But even then, biology is complex and not always at a binary. Intersex people exist, and lots of people are not 100% male or female.
The difference here is that gender is an arbitrary thing to ascribe to people based on those biological differences, but in reality that can wildly differ between cultures and in history. For example, most things about "womanhood" or "femininity" is not directly linked to having tits and a vagina - how is the colour pink related to your biology? How is makeup or wearing a dress related to your biology? They're not.
Some cultures in the past even had more than 2 genders, or their idea of gender was vastly different to ours, so it's not even a new or particularly controversial thing. Even in Europe, gender has changed massively over the years - have you seen how feminine a lot of renaissance men looked for example? Pink wasn't a "girly" colour until after the 1940s!
Then of course, you have to consider other animals, who literally have no concept of gender at all. Are female dogs naturally prepositioned to dresses and dolls?