Why is anyone surprised?
The desire to image ladies sans clothes has been a driver of technology since day one... man paints on cave wall, man paints naked lady on cave wall. Man invents photography, man photographs naked ladies. Man invents 3-D photography, man makes 3-D photos of naked ladies. Man invents moving pictures, video recorders, DVDs, the internet... and man puts pictures of naked ladies on all of them.
And in all that time, ladies of negotiable short-term affection have also found it convenient to advertise their wares... are we surprised that they're on such sites?