It's suprisingly easy - back when fandom was in mailing lists, and then LiveJournal, "slash" (yaoi as a term was anime fandoms, slash was generally western media, but it's the same thing) fans were strictly separated from the "het" or "gen" fans. You could not post any fic with a same sex pairing to main comms or lists, and the few that would allow it required specific warnings. This lead to spaces that were a bit outcast from regular fic spaces, so there would often be quite a few misfits. It was still almost entirely female, but it also tended to have the lesbian and bisexual women along with the straight women. There were occasionally surveys or discussions about why slash (especially with lesbians), and the appeal was usually a combo of main characters being male, important relationships being between male characters, not wanting to deal with gender roles and feelings about women and her relationships. Bunch of reasons - but useful background for following this progression.
Quite a few people took their writing seriously enough to care about trying to write "realistically". This is where problems come in. There was a site by a gay man giving advice on how to write gay sex for slash writers - Minotaur or something? But it was treated like a Bible, and created a huge number of tropes that were then adopted by subsequent new writers years and years on. So twenty years on, stuff from that guy and his site memetically influenced how these women write.
LiveJournal was very segregated- more like reddit than Twitter or tumblr, with various communities large, small, public, private, popular, or niche. You could have incredibly filtered friends and spaces. Then LiveJournal fucked up multiple times, one of which was bowing to some Christian group who was pushing for the site to be child friendly. Lots of journals, comms, etc were deleted. By the time the company was eventually sold to a Russian company, fandom was moving to tumblr and Archive of Our Own existed for a safer place to post fic without censorship.
But tumblr doesn't allow for communities or closed spaces. It's part of the reason all of their weirdness is contagious - before people were a bit more bricked off, so things didn't spread the same way. So, you have a weird niche of women already, some of whom cared about social justice stuff, mix it with tumblr and things spreading like a disease and everything is hugely amplified. A gay guy bitching about how women were oppressing him with their stupid, unrealistic girly fics would actually get attention, and since SJW was the standard on tumblr, people couldn't just say "I don't care" and go back to doing what they wanted. No locked comms to keep people separated from shit that bothered them, everything spreading outside of designated spaces, and then the shaming would start. Now, plenty of people would still just ignore this stuff, and keep on with what they were doing, but as time went on and more and more young people who are easier to influence joined, it would become more and more of an undercurrent - women writing their little stories were just kind of weird. Now they're oppressing actual gay men, or "appropriating" their experiences, objectifying them. Blah, blah, blah social justice nonsense.
This is also when the first "trans men" would start popping up. And I have to emphasize here how much women in these spaces would suck the dick of any "man" in the hen house. If incels were smarter and less insufferable, they would have learned to write some gay Supernatural fic or something, and they would have had way more female attention than any fedora would give them. But since it's the internet, any woman who wanted could basically be a man, because no one would really know if you were only communicating via text. There's a person called copperbadge who has lapped as a man for twenty years - I think they're officially "trans man" now, but before there was no caveat.
Fast forward, and things are way more. Rigid. Fraught. Tense. The SJW thing has successfully permeated, and tumblr starts having more and more variations of "you shouldn't write this if you're not a gay man". (There's also the "antis" and their really extreme inability to separate fiction from reality - if you like a "problematic" character or ship, you're obviously a bad person.) That's not going to work on people with a firmer grasp of reality (and that space on the internet is basically unlimited, so literally no one is stopping gay men from writing anything they want, and not reading shitty girly fanfics), but the younger, less confident? They are the ones who buy that shit. And trans was the new oppressed group on the block. So the fujos start going trans, and start perpetuating the "you shouldn't write this if you're not a gay man. You're oppressing/objectifying/etc us True and Honest gay men".
Because nothing is quite as satisfying in the oppression Olympics is getting to wokescold people, and even pretend men are very good at female social pressure, you dial up the shame. And the weak, young, autistic now aren't just weird for liking some stories with dudes making out - they're horrible people. But! Tumblr also says that trans people are what they say they are. And dysphoria isn't needed to be trans. And maybe I like reading about guys because I really am a guy? And it's the internet, so I can be a boy if I want to.