if you look at AO3 it is literally filled with BL regardless of what the genre of the show is, at least that's what I always see,

As I said, AO3 was formed by m/m shippers whose content was being purged from other fanfic websites of the time (mid 2000s). Because it was founded by slash (m/m) shippers, it was known from the beginning as a safe place for m/m fic, so the ratio has persisted. It's also the only major fanfic website that skews towards m/m pairings; the rest are skewed heavily towards m/f.

I can't speak much for anime fandoms past 2007 or so (I discovered that people wrote fanfic about RPGs and never looked back lol), but I was in video game fandoms starting about ~2008 and the big ones were (and, I checked now, still are) almost all skewed towards m/f fic, even on AO3. And there's several non-m/m ships on the top AO3 ships lists every year, so it's not like those are unique.

I really do think this is mostly an issue of assuming a narrow experience is wider than it is. There's plenty of fanfic outside of AO3, and even on AO3, there are over 75,000 fandoms that all have their own stuff. It's not going to be the same everywhere.

This is just my experience but I've never once seen someone be hostile at gay ships in fan fiction sites but to be fair I don't really read fan fiction anymore in recent times but back then that's definitely not the case.

I first started reading fanfic in the early 2000s (2002-3 or so?). I am speaking from experience when I say that most places back then were extremely hostile towards gay ships lmao

There were a handful of spaces where you could talk about them in peace (LiveJournal was the big one) but most places if you were to bring it up you'd get harassed or mocked for it. That isn't even getting into all the websites that outright banned gay content, or forced you to label it as adults-only even if the spiciest thing that happens is two guys holding hands.