I don't want to derail the thread too much, but I really think fanfiction is the key to unlocking much of the FTM explosion of the last few years. (Specifically the 'gay boy' variety.) Obviously the rise in misogyny and porn are factors. They're what these girls are fleeing when they try to escape into 'manhood'. But their whole concept of manhood comes from fanfiction. It's one of those generational divide things, where if you've been in that environment, it's clear as day what's happening with these girls, and if you haven't, you only half understand it.
We've got a whole generation of girls whose sexuality has been moulded by AO3, in the same way their male peers have been moulded by Pornhub. These girls have all these warped ideas of what gay men are, and that's been their outlet for masturbation, from a formative age. It's absolutely a factor. It's driving their sexual tastes and altering their self-image as sexual beings. In many cases, these (young, female, virgin, autistic) writers will start consuming gay porn, in the interest of writing more 'accurate' sex scenes. And then because it's much easier to self-insert into fiction than other types of pornography, even acts that might have repelled them if just seen on a screen, start to feel alluring. Like choking. And so they slip on down the slope.
The thing is, I'm not anti fanfiction. I'm not here to laugh at it. At its best, it can be great. It's a creative outlet, like any other, and a predominantly female one. But the fan writing community has the same problem as the gay community. Because they're so used to being mocked and put down by outsiders, they place a high value on being welcoming to anyone inside the tent. If you shame someone in the fanfiction community, you're practically JK Rowling. You're practically Hitler.
The result is that there is no quality control, and no real attempt to shield younger members of the community from being exposed to inappropriate content. The mentality is very much that if you chose to ignore a tag and click in to a fic that has an explicit warning, that's on you. The author did nothing wrong in posting their father-son incest noncon vore fic - you're the problem for clicking on it! Which is fine, but only works if you ignore the fact that much of the site's user base are not rational adults making rational adult decisions. They're kids. Kids are curious and impatient and often won't understand what they're clicking through to see, until it's too late. They may not even understand what all the different tags mean, if they're new to the site. It's a very disingenuous argument.
The AO3 model is the equivalent of giving your 12 year old the tv remote and saying "I'm trusting you not to watch that 18 rated horror movie". Then being shocked when they give in to curiosity and end up having screaming nightmares for the next six weeks. Like, really. What did you expect the 12 year old to do? Are you really going to blame them in this situation?
AO3's main selling point has become that they will not enforce censorship on their writers, no matter what. Some other aspects of the way the site is structured have exacerbated this, but basically, it's resulted in a race to the bottom. The site is swamped with poor quality works, because the most shocking and sexual content is rewarded with clicks. So authors keep churning that out, and burying the better stuff that updates more slowly.
A frequent complaint in fanfiction spaces is how hard it is to find "gen" fics - stories that are just about the world and the characters. Or stories that aren't solely "fluff" (tooth-rottingly sweet happy endings) but don't contain explicit sex scenes either. Both these things used to be common before AO3 became dominant. On most other fanfic sites, there was a limit in how explicit you could be, and fics would be removed if they failed to meet that standard. Mature rated fics were hidden from the main page of works, and you had to opt in to see them. A sex scene might be permitted in a fic rated as mature - but things like BDSM, bestiality, and most of the 'kink' content that now proliferates on AO3, would not. Obviously there were times when this safeguarding butted heads with censorship. But it made the entire space feel much safer for teens.
Given that teen girls are probably the biggest audience for fanfiction, I'd say that matters. Teen girls are being affected by the lack of safeguarding in this space. I respect the aim of AO3, to be a user-funded archive that preserves fanworks. I even respect their anti-censorship stance. But you can still have that content, if you put it behind a wall where only users who know where to look will find it. It shouldn't be accessible to visitors who don't even have an account with the site. It shouldn't be all over the main page for a fandom, with the responsibility resting on the (often underage) viewer to scroll past or filter it out.
That was a bit of a digression, sorry. But this is a fascinating subject, and an overlooked piece of the puzzle when talking about female transitioners. I'd love to see studies on it. Helen Joyce touched the tip of the iceberg in Trans, but if she ever wanted to write another book, there is so much to explore and unpack in this topic.