Part of what drives Porn, what makes one setting or situation more popular than another, is taboo. Having just enough of a sense of “naughtiness” in a situation that it makes it more exciting, without going so far as to be offensive to the senses and take the viewer out of it. A generally successful way to do that is to take a normal situation that people are familiar with, something that wasn’t sexy, and make it sexy and pornify it.

As an example, imagine you’ve… ordered a pizza, or had some sort of workman come to repair something at your home. Pretty normal, right? But have you ever paid for that service with sex? Normal, relatable situation, made sexy. Now this is a cloche these days because it’s been done to death, but at it’s advent the “Pay for thing/get paid for thing with sex” trope wasn’t just new, it was Thrilling. It also stops short of presenting the impression of full on prostitution, the goods or services were not presented with the intent to buy sex, which makes it feel less… dirty, to the average viewer. It’s taboo, relatable, but not entirely repulsive.

Now, the “Step” concept is the new hotness. Lots of us, maybe most of us, have step-family, so it’s widely relatable. Also we generally don’t seriously consider sex with step-family, making it taboo. Finally it always stops at STEP family, not blood relatives, so it’s arguably not really incest, meaning it’s not so broadly repulsive. It makes a condition that we can see ourselves in, it’s even more naughty than just sex, but it just manages to dodge the most objectionable subject that it’s so close to.