It's simple: we are just animals (a litter more inteligent than most animals) and we think a lot about survival. And, we accept it or not, making sex and reproducing is, basely, survival.
Even we admit it or not, the sexual part of each of us defines the most people, and the weak ones will run to get every chance they will have. It does not matter that a lot of this chances are not realistic, they will just go for it... the mind is a very good trickster and it can convince the humans to do almost anything.
You want to make some money? Get some chicks to get undressed, make some videos with a couple or some treesome getting wild and they will come. Do you know the motto from some Kevin Costner movie, don't you? "Build it and they will come". That's very true when dealing with porn: you just need some picture and every stupid will come licking his dick.
