> Isn't biological sex based on biology

Butler takes the post modernist stance that "biology" as a group of rules and ideas is inherently made up and therefore meaningless.

They do not believe that because we observed reality and constructed these rules and ideas from these observations, that it means there is any validity to these rules.

The central tenant of post modernism is that the human subjective makes any sort of objectivism impossible (some even go further and claim that objectiveness is impossible itself)

Of course, this is why these kinds of ideas are limited to philosophy and other related fields - scientific theory is predicated on the idea that objectiveness does exist and is achievable to some degree.