Photographs of the model standing, and lying down, with the penis relaxed
In the standing position it can clearly be seen that the testicles protrude forward of the gap between the legs and the slack penis naturally hangs well out, to the front of the body.
The question is where does the penis lie when the model is lying flat on a hard surface.
The easist way to find out was to lie the model flat and photograph him. The result is shown below.
In this position although the testicles tend to drop down towards the body, they form a support which rolls the flaccid penis to one side, causing it to drape over the legs and lower stomach. Even with the legs slighty open the penis is higher than any of the surrounding flesh and it naturally lies in a position which will bring it into contact with any covering cloth.
The above photograph demonstrates this effect. The model's penis was not adjusted or posed for these images, but simply photographed to reveal the position it would settle into naturally under these circumstances. These photographs reveal that the penis does not slip down between the legs and so if the body was flat would come into contact with any covering material.
I have assumed that the image on the Turin shroud was created by an unexpected natural process, so whoever draped the body with the shroud would not know they were creating an enduring image, and would have no reason to manipulate the penis of the subject to avoid it showing on the shroud. Under these circumstances the penis of the subject could be expected to lie in its natural position.
This simple observation suggests that if the image on the shroud was formed by the body of a naked man lying flat on a hard stone surface then there should be an imprint from the penis on the cloth.
This hypothesis was then tested by experiment.