Supposedly, it was accidental.

Early on, porn sites were less moderated, and thus more friendly to niche kinks.

But this meant people randomly looking would run across those niche kinks, click on them because it's effectively shock bait, which skewed views. The porn makers saw this increase in views, figured they hit a gold mine, and incest porn took off. As the internet got more moderated, the taboo kinda pushed regular incest to the side, with some sites outright banning it eventually. This left step-porn stuck in the dryer, and the effect spun out of control.

Mind you, this is from some random article I read a few years ago, so it's second hand and unsourced, so grain of salt, etc.

That being said, it jibes with what I've seen over the years. I've never been a big porn user, but even a dilettante like me could see trends in what sites were showing up front. I wanna say the step-porn started showing up in otherwise unrelated searches maybe eight years ago? And it was visible as far back as something like 2005ish as something you'd run across on porn sites.

Mind you, I don't have any issues with step-porn or regular incest porn; it's almost 100% fake anyway. I don't kink shame, even with the really "gross" stuff like scat, as long as I don't have to see it unintentionally. If it was real incest porn, I'd not want to see it, but as long as it's consenting adults, I ain't hating. Step-porn that's real is just regular porn where the performers already knew each other well, it isn't incest in the important ways.

But, dude, that you've never run across any of the ugly stuff just shows how much the internet has changed. There was a time when just being in a non sex related chat room could expose you to some pretty bad stuff. Shit, I still can't get some of the things I saw back in the very early internet days out of my head.

You'd just be talking about random shit, maybe a movie or whatever, and a proto-troll would dump images of kids, or gore, or both, plus whatever else they had saved up. It took a while before videos could be done like that, and thanks be to the tech gods that there were better tools to reduce that kind of fucked up shit on a given chat. But you'd still get clips dumped in of things you can't erase from your head.

By the time commercial porn sites came along, they were able to limit the worst of it, and did. But before that, it was a risky thing to run searches that included some words that would pull hits from porn sites that were either unmoderated, or were intended to host the bad stuff. Ffs, there was a time when trying to figure out how to light a photo you intended to take of a kid was just horrifying.

As much as I hate Google, they really did reduce that, then eliminate it to the point that it really isn't possible to find such things on purpose, much less having it show up like that. Well, I assume so, anyway. Afaik, all that shit went underground, and you can actually search for the Nabokov novel without getting your eyes and soul tainted.

Jfc, I really am glad y'all younger netizens didn't have to go through that era.