Well, dunno if you've noticed this, but the moral crusaders hate incestuous roleplay too. You used to be able to search for that stuff on Pornhub, XHamster, etc. but nowadays it's scrubbed away and you can only find stepfamily roleplay stuff, if you're lucky, unless you go to more... specialized sites. You get similar issues with finding rape/forced/non-con roleplay porn, used to be you could find that stuff easily but now it's also gone from mainstream sites.
It's not any kind of principled stand, the truth is they hate all media. It's pure strategy, they just attack whatever is on the furthest edge of social acceptability until it becomes no longer acceptable, then they move on to attacking whatever is now at that borderline. Once they finish their crusade of scrubbing all kink and queerness from the internet by lumping it in the same bucket as **********, they'll move on to scrubbing all vanilla sexuality too. (They're already laying the groundwork for this with their 'Every single girl posting their boobs on OF is actually a sex trafficking victim' narrative.)