There are special payment processors catering to porn/etc, but the problem is that they require extremely large collateral and have other unusual requirements that are untenable to new businesses. Most probably they won't even talk to a startup (I tried, didn't get a reply). There's a reason why startups like OnlyFans used normal payment processors and tried to convince them they're not porn instead of beginning with these specialized ones.

Edit: BTW, a large portion of seemingly independent porn sites are actually run by only few companies. Especially the Xvideos corporation.