Gender studies did indeed start out this way, dismissive of notions of "biological sex," until the case of David Reimer threw a bucket of cold water on those second-wave feminist theories. David was the victim of a botched circumcision as an infant. His doctors decided to turn an outie into an innie, constructed a vagina, and told his parents to raise him as a woman. He was given "vitamin" pills (actually estrogen) and forcibly socialized as female, but it didn't take. He identified as male throughout his life, transitioned as an adult, eventually discovered the truth his parents were hiding and finally killed himself.

This was an early factor in third-wave feminism accepting gender identity, which is why almost all millenial and gen-Z feminists accept trans and intersex people and why TERFs tend to be older (or deceased, good riddance Janice Raymond.)

I've found that the left tends to distrust science that may contradict its ideology, and the right tends to distrust experiences too alien from their own. In Reimer's case, the science had a human tragedy at the center of it which bridged the gap and led to change.