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Sonic fans aren't really that weird. I mean, there are a lot of them, sure, but I feel like there's an undue focus on them just because it's low hanging fruit.
You can point to the inordinate amount of porn or bad fanfiction or whatever, but I think that's just more a result of Sonic's popularity. There's a lot of fucked up Harry Potter fanfiction out there, too, but you don't see people hounding them constantly. But that's because Harry Potter ended on a high note, whereas Sonic's been gaming's designated whipping boy for some time now.
A lot of that is deserved, mind you. There have been a lot of bad Sonic games. But that doesn't change the fact that there are people out there who cross a lot of lines because that kind of extreme humor is funny.
Part of that is also just the size of the Sonic community. It's a big place, and not an entirely unified place. There's individual communities for individual pieces of Sonic; you have places exclusively dedicated to the old Saturday Morning Cartoon, places to discuss the comics, the new games, the old games... etcetera. And each Sonic sub-community is usually composed of a unique group of people with very little overlap.
So the creepy Sonic fans? Other Sonic fans hate them, or at the very least, make fun of them.
It's also one of the reasons why Sonic fans can't ever seem to agree on what they want from Sonic games, because everybody is in it for a different version of the character, and there's no universal consensus on which one is the "right" portrayal.
As for why the crazy Sonic fans are crazy, well, I feel like that's a two-fold problem.
Number one is how Sega treated the Sonic franchise. I don't think people remember what a juggernaut Sonic was in the 90's. He was EVERYWHERE, on EVERYTHING. And then the Sega Saturn came along and Sonic the Hedgehog struggled to maintain relevance.
For about five years, if you were a Sonic fan, your choices were:
Sonic 3D Blast on the Genesis and Saturn. Eugh.
Archie's Sonic Comics, from an era where the now-hated Ken Penders was in full force.
Sonic R, which was from an era where Sega was doing very limited production runs on Saturn games.
Not great. But thanks to the comics, there became a growing focus on Sonic the Hedgehog's story. One thing lead to another from there, leading to a lot of that weird fanfiction.
The second part is that Sonic the Hedgehog, especially nowadays, is aimed squarely at what we'll call the "puberty demographic".
Shit gets real weird in puberty. I would expect that most of the people posting in this thread are of the age where they're lucky none of the shit they got up to in puberty is logged on the internet. But it's something everybody goes through. But, again, it goes back to what I said earlier about Harry Potter - or, really, any "puberty demographic" franchise. You can find Birdo Inflation Porn if you punch in the wrong Google Image Search terms (or, you know, any Birdo search terms, apparently).
But that's just the internet now. There are whole websites designed to catalog rule 34 junk. Get used to it, I guess.
