I didn’t mean to suggest that sex is easy to define, just that characteristics like chromosomes, gametes and the like cannot be changed by transitioning. There are obviously things we can change (hormonal makeup, appearance), but it doesn’t change the underlying biology that you are born with.

Definitely agree that the whole concept of sex becomes shaky in some circumstances - but it remains a concept which has value in fields like pharmacology. There are outliers, but there are also a huge number of people for whom the basic male/female categories apply.