As of January 2024, of the most popular 100 videos on Pornhub, 4.1 billion views were for step-incest videos and 3.3 billion views were for all other videos combined.
I think everybody is overthinking it, honestly. This is probably a UX problem. At some point some of these taboo reasons led to a relatively minor trend of roleplaying that situation, that made “step” a SEO bonus, and since you can claim any porn fits the genre, even retroactively, it also became the default way to label a porn video in algorithmically served free porn sites. And now it’s probably just convention, or tradition or whatever.
I think everybody is overthinking it, honestly. This is probably a UX problem. At some point some of these taboo reasons led to a relatively minor trend of roleplaying that situation, that made “step” a SEO bonus, and since you can claim any porn fits the genre, even retroactively, it also became the default way to label a porn video in algorithmically served free porn sites. And now it’s probably just convention, or tradition or whatever.