Chatting with a Therapist back in the 90's, I mentioned whenever I was in an art store, I felt a need to evacuate my bowels. The therapist said grocery stores did that to her, so I came up with my own theory:
That which we find pleasurable stimulates certain chemicals in the brain, for example, adrenalin, which stimulates, among other things, muscular action. The bowels are muscular in nature, moving food and fecal matter through the alimentary canal through a wave-like muscular action called peristalsis.
Hence, pleasure increases peristalsis which increases the need to poop.