Any idea what is the actual harm of seeing a human body part? Especially in non-sexual context? How does it wreck a child if they see that "this man has a penis

Any idea what is the actual harm of seeing a human body part? Especially in non-sexual context? How does it wreck a child if they see that "this man has a penis like my father does". At least in Finland we go to sauna and naked swimming in lake with grandparents and friends of parents starting from age around 1 year old. There would be at least a few million pedophiles, alcoholics and mental cases due to that in our population, if nakedness would be such a scary thing. As a child I probably saw more breasts in women's saunas than in my adult life :-) Just asking this in a curious manner. If people find exposed body parts, the ones they also have, somehow disgusting, then it is quite understandable to me.