Part of what drives popular porn is a sense of taboo, enough to make something more exciting without going so far as to become repulsive. An easy way to do that is to take a normal, relatable situation, and sexify it. That’s why Step-Incest porn is so popular right now.

Back in the day, this was the service worker / payment trope, done so extensively that it’s cliche now. Plenty of people will hire a plumber to do some work in their home. Normal, relatable, but what if that then leads to sex with some lines about “cleaning pipes”? Suddenly this mundane thing that people can relate to becomes sexy. How about ordering a pizza, but you discover you don’t have any cash, so you offer to pay with sex! Another normal situation made progressively more exciting. It all edges close to prostitution, paying someone for sex or exchanging sex for goods or services, but it neatly skirts around that somewhat more taboo, and certainly at the time objectionable concept.

Now, it’s step-incest. Lots of people have step-siblings, it’s very relatable, but most of us wouldn’t really consider sex with them. That takes the relatable situation and makes it more taboo, more sexy. This stops short of the real taboo though, actual incest. It’s almost always Step siblings, because that gets close enough to the forbidden to be more thrilling, without really triggering that ingrained repulsion in enough people to make it worth making.

Step-incest porn is popular because having step siblings is relatable, but something most people wouldn’t normally relate to sex, while allowing porn to edge closer to the forbidden taboo of actual incest without actually crossing that line.