The picture below shows a rear view of the back and buttocks of a human being sitting in a chair.
The back and buttocks are outlined in the next picture.
That outline forms the shape of a penis. It will be easier to demonstrate that similarity if the picture is first flipped vertically.
This picture compares a real penis to the flipped picture of the back and buttocks.
The shape formed by both buttocks is similar to the shape of the head of the penis,
while the orientation and shape of the back are similar to the orientation and shape of the shaft of the penis.
The final picture outlines the shape of the penis and the shape of the back and buttocks to highlight their overall similarity.
