Porn wasn’t always customized for viewers with niche interests. From the late 1960s through the mid-1980s, porn in the United States was primarily available in the about 1,000 select theaters across the country that specialized in “adult entertainment,” said Peter Alilunas, a porn historian at the University of Oregon. At first, these theaters screened what today is known as soft-core pornography, such as a series of 10-minute films of people taking off their clothes. Over time, these films become more naturalistic, and grew to develop narrative, plot-driven elements, including depictions of two people having sex that were more realistic than what is popular today.

What Alilunas calls the “theatrical era” gave way to the home-video era of the late ’80s and early ’90s. Porn consumers had more choice—movie stores sold and rented a variety of pornographic videos on VHS—but they were somewhat hamstrung by what they were willing to purchase in public.

The internet has introduced a “stunningly massive algorithmic system,” Alilunas told me, allowing consumers to choose from thousands of tailored categories and subcategories catering to a wide variety of fantasies. Since the early 2000s, one company, MindGeek, has taken a dominant position in the industry. It manages the vast majority of the biggest porn websites, and it's been using its reach in the marketplace to compile and categorize a massive database of porn videos. As a result, viewers can find exactly what they're looking for—whatever that is. “They’re doing the same thing that Netflix and Hulu are doing,” Alilunas said. “They’re just doing it for porn.” The result, Lehmiller told me, is “a catalog of endless diversity.” Almost anything can be found with a few keywords and the click of a button.

Because companies such as MindGeek “push what people pay for,” said Jennifer Johnson, a sociology professor at Virginia Commonwealth University who specializes in pornography, they might drive viewers toward more niche material. Most people don’t pay for porn—their viewing is underwritten by advertising. But porn websites, Johnson told me, cater most to their paying customers, giving prime placement to their tastes. “The porn that is most easily accessible is that which is most stimulating and interesting to the most active users,” Johnson said.

Humans, Lehmiller told me, also have an intrinsic desire for “newness” and “novelty.” Viewers of pornography tend to go through cycles of different pornographic interests, Alilunas said, watching a lot of one type of porn for a while before moving on to something else, and then sometimes circling back around. Lately, Lehmiller said, there’s been a steady stream of porn that incorporates elements of fantasy and science fiction, a trend likely spurred by evolving animation and CGI techniques. (Vampires and robots, he told me, have been especially popular.)