There's an interesting historical reason. A while ago, some porn website (websites?) were optimizing what genres they recommend by default. To choose which categories to boost, they looked at their search data and found that there were lots of searches for incest porn, so they concluded that this is what people are really into.

Turns out the way most people use porn sites is to scroll around a little and watch whatever catches their eye. People tend to only use the search bar if they're into something they know they're not gonna find by doing that, so paradoxically they concluded that certain genres were popular because they were unpopular.