Jim, I think your body has an ability to balance over the BB, and when you do so your butt has chosen its place in the geometry of the bicycle. The saddle has to be placed so it meets your butt and supports your weight by contacting your sit-bones.

If you saddle is too far to the rear your butt will migrate forward as you ride. Then you'll get your weight supported by the nose or narrow part of the saddle, and the sit bones will not be holding your weight, your perineum will. THAT will hurt, possibly a lot.