I think the mechanism in question is more on the brain side. Where certain sets of nerves are processed, if some are missing that area of brain simply adjusts the input strength of others. I suspect adult amputation is different from amputation of a newborn since the brain elasticity is so different. But all we can do it make educated guesses anyway since we can’t do controlled experiments. Studies involving watching brain activity can only go so far to really reflect experience. So we can’t know. I’m just pointing out that the common sense approach you indicated isn’t matched by some clear data. So it’s not cut and dry. It could even be that men circumcised at birth experience more sexual pleasure.