I actually have the full video that this scene is from and it's more of a loose documentary of sorts (that was made in the late 70's/early 80's I believe) on the photographer and a few of the hugely hung models he shoots for porn magazines. It also included a session with Long Dong Silver - the Bermudian with the 18" cock (which also was apparently later found to be partially enhanced by a prosthetic). Since this was from back in the day when porn magazines were big, a porn photo shoot wasn't synonomous with a video sex shoot, so I don't think it's that unusual that the models in this clip were only posing and not actually having sex for the camera - which probably made it easier to "pull off" wearing a prosthetic as well...