I feel like 100 endocrinology papers in I finally have a good understanding of what we as transfeminists mean by "sex is constructed": endocrinology studies start with a subject population already split into "male" and "female" and take this split and how it is made completely for granted. Nobody ever includes in their methods section how they decided which patients were male and which were female - even in papers where this is a relevant question! where reading the text of the paper naturally raises this question! Instead they are just using these social ("received," "unscientific") notions of male and female and then analysing the data along that split. They are using biology to find a possible justification for the existing social categories - not using biology to try to discover "natural" categories, and therefore this biology can fundamentally never prove that sex is a natural dichotomy! That is an assumption, not a conclusion.
To make this more concrete: I believe that the distribution of estradiol levels among non-pregnant adults who are not currently ovulating is unimodal, not bimodal (so not "sexually dimorphic"). But I cant find data to prove this because nobody is publishing a histogram of "human estradiol levels": they only ever publish separate graphs or tables of "male levels" and "female levels"! If biological sex is supposed to be a discovered distinction from differences in hormone levels, why has nobody ever published a study with a histogram of "all adult estradiol levels" in order to point out that this distribution has two peaks, and so we should categorize people based on what peak they're in? Because the existence of two distinct sexes is assumed going into biology, and is not a conclusion of biology.