Mario Salieri was shooting a pornographic movie in a lavish Prague villa two decades ago when he first caught sight of the computer geeks who were about to upend his industry.

“The owner of the villa asked me if we could offer a sandwich to a young computer programmer who had been renting a room,” says Mr. Salieri. “The boy was pale and visibly hungry.”

A few years later, Mr. Salieri discovered that “the boy” had bought his first Rolls-Royce Phantom. Like other coders, he had made a fortune selling advertising on the early free-to-watch porn sites, which today attract hundreds of millions of visits every day.