I also am old enough to remember when gender was a social construct.
It's not a thing that ceases to be. Nobody outside ultraconservative groups claims that women are inherently better at dishwashing or worse at driving than men, or spontaneously prefer pink to blue. The cultural construction of "manly" and "womanly" behaviors and aptitude is too well documented, in its processes, relativity and outcomes.
The latest development is only that we used to contrast gender (the cultural attribution of "feminity" and "masculinity" to traits, behaviors, activities, etc) with biological sex, assuming that the latter was a simple binary thing. We recently found out that biological sex itself is a mess, that it's determined by a lot of physical/chemical components that can be much more contradictory than our simple idea of clear-cut determination by either XX or XY sexual chromosome pairs. Depending on your sensitivity, the implications are hilarious, or terrifying, or beautiful, or outrageous, but anyway there we are. And it's a completely separate matter than the question of cultural gender (apart from the aspect of "what do cultures do with that", which is always "paint their local idea of gender over it").
So yes, we found out that biological sex, itself, was also a social construct, in the sense that it was medically categorized as binary, and medically classified on "obvious" physical traits (with its even physical inbetween cynically denied through surgery, in order to preserve our imagned dychotomy). There is a biological continuum that medecine has no choice but taking in account from now on. We can't unlearn what we discover about cultures and about bodies.
But the super embarrassing thing is that, for ages, the concerned people "knew". And were mocked. Now that we know that there is something more complex than we believed, underneath sexual idenities, and that we were unfairly denying this in front of people who "felt" it because it was their own bodies, inhabited by themselves
, we learnt to be extra careful and to fucking listen to what people say about themselves
. It took us some time.
So yes, self-determination is important. There are two different layers at stake. And nothing matters mote than what people feel about themselves
(especially as, fucking hell, why should OTHERS care,
what do OTHERS have to lose,
in comparison to the person who has to live with their own body). So people can feel like they belong to the other sex, and it should be investigated as a serious claim, with the person's own wellbeing as a priority. Other people feel like they belong to the other gender, and heck, why not, it's their life. Other people belong to their assigned sex and gender, yet feel sexually and/or emotionally attracted to the same sex and gender, and again, we have nothing to say against it
, zero legitimity in ruiining their lives on the ground that they are counter-intuitive to us, or contradict whatever archeological table's diktats.
It is early. We are living in societies, within a species, which self-awareness and self-understanding is in perpetual progress. We adapt slowly to what we realise about ourselves. So of course, jerks will deride, invisibilize, destroy transsexuals, just like they did to homosexuals, just like they did to so-called "mixed-race" people, or to "inferior races" behaving as humans, or to women and men transgressing the lines of gender traits/activities/attitudes assignations. Jerks are jerks. Society outgrows them. Ridicule gets replaced by the ridicule of finding something ridicule. It doesn't happen overnight. It doesn't happen evenly. It happens. And nowadays, ladies can have short hair and wear trousers without being demonized. And -in many countries- men can fall in love with men without the crowd to point at them and laugh. Black skinned people and white skinned people can marry without being stoned to death for "betrayal". Transgenderism is going the same route for the same reason.
Just not at the same speed everywhere.