Part of the problem is that there's too much under the umbrella to get a clear picture. This is how I understand it currently.
- Most people think of (and are sympathetic to) trans, once known as transsexual, as "gender dysphoria", a strong difficulty with the sexed body, akin to the rare cases of Body Integrity Identity Disorder where people can't cope with a limb or have a strong desire to be disabled. GD has all the societal issues of gender and sexuality on top to consider, as well as any co-morbid mental illness, which may preceed or arise because of the GD. From what I can tell this is a small minority who benefit from psychological intervention; a recent study suggested sex reassignment surgery was not always the answer. Not all of these people go through with SRS; some have insight that they are their natal sex and do not presume to redefine the sex they wish to become. Others want in to women's spaces to validate themselves. Sometimes very young children are thought of as in this category; personally I hold a high degree of scepticism on this point.
Trans gender - in the case of young people, particularly girls - is more akin to self-harm and anorexia in that: 1. there's usually a social contagion element, from clusters of friends or the internet 2. there's often co-morbidity of mental health issues 3. There's often an element of sexual abuse or neglect 4. There is usually a desire to "identify out" of the situation (of being a girl under patriarchy, or a changing body, or even to please an adult) by being different, changing the material reality of one's body. Note I'm saying "often" or "usually", not "always". It's complex. There's also vulnerability issues - a significant number of girls particularly have extra factors such as ADD or autism to consider. Another part of the puzzle is sexuality - a hugely significant number of girls detransition and come out as lesbian, so trans has been a way of denying/delaying/hiding sexuality in a threatening/unaccepting atmosphere. Sadly many of these kids get put straight onto the medical pathway with no MH/SEN support and the potential for severe damage is very high. This is one of the big issues for many women here.
Then you've got your AGP (autogynophiliac) types - usually older men that benefit from all their male privilege growing up, then put on the caricatured, stereotyped costume of woman because they get off on the idea of themselves as female, submissive, etc. Often heavily into porn and other paraphilias. The trans widows thread is the best place to look for info on this. These men are often the loudest campaigners, banging on about surgery for children whilst resisting it themselves. They're many of the ones demanding access to women's spaces as a power trip. Peel back the dresses and make-up and you find men's rights activists, pornsick fetishists and misogyny running through like a stick of rock. These are the men we used to think of as cross-dressers and transvestites - some still harmless, but the loudest have soaked up all sorts of ideas, again via social contagion.
And then you have the ones hanging on to the coattails of the movement - the paraphiliacs (adult babies, furries), the paedophiles, the rapists - who will happily don heels and fake tits if it gets them closer to to their targets. You have kids (and grown-ass adults who should know better) trying to be non-binary or queer who've just drunk the pomo kool-aid and are trying to be special or play the oppression Olympics out of narcissism (they make me want to sing Common People very loudly).
All of these groups have crossovers. There's no firm line to demarcate where a young boy with GD becomes a man with AGP. We don't know the effects of modern porn (though we can speculate accurately). We don't know if the 13yo girl with the breast binder would have been cutting ten years ago, or starving herself twenty years ago (but it looks likely).
We do know that adult trans brains resemble their natal sex more than their required sex. We know that there are distinct brain differences in the region of trans brains that deals with body perception - but we have no idea if that's a cause, or an effect. www.nature.com/articles/s41598-017-17352-8 - it probably needs it's own thread, I'd like to make sure I'm interpreting it properly as my knowledge of scanning technology is lacking.
Tl;dr: sorry for the epistle, trans is not the same as sexuality.