This is one of those convenient statements that we encounter in daily life, a truism, or intended truism, that has no basis in fact. These are common in our daily discourse with fellow humans.
Starting in elementary school, (grade 7) we were required to change for phys-ed, and shower afterwards. The change room and showers were communal, and wide open. For the first time in my life I encountered other boys naked. The surprise for me was that I and only one other boy had pubic hair and man-sized equipment, which seemed to shock most of the other guys, to say the least. Of course, we learned that puberty travels at its own speed, and varies by individual. And that by secondary school (grade 10) we were far more alike than we were different from one another.
Throughout elementary, secondary schools, and university, I came to realize that there is a variety of human body parts, bone structure, muscularity, height, width, proportioning and symmetry, to name but a few distinctions. The point being - that we all knew, and had grown up with knowing what our bodies looked like. And that the differences were always present, but hardly important. When I hear of males in school who never shower, or drop phys-ed as soon as it becomes elective, I shake my head. When I see young males wearing multiple layers of clothing as loose fitting as possible, sometimes falling off ... I can only conclude that they are scared of their own cocks, and seem to believe that all of this excess material hides thier hardons. What can be more masculine than an erection ?
When getting fitted for my first bespoke suit, I learned that even on the same body, one leg is longer, one foot is larger, the hips nor the shoulders are exactly equal across the body, and of course, I had observed by then that males wore different kinds of (or no) underwear, of different fit and style, and seemed to very creative in "arranging" their equipment inside their clothing.
Similarly, during these years, (I date myself here) absolutely everyone wore Speedo bathing suits, and posters of Mark Spitz in his, proudly displaying his array of gold medals across his chest, were to be found in many teen male bedrooms, alongside the Ferrari, Skiing, and Playboy centrefolds.
It strikes me as strage to believe once you've seen one, you've seen 'em all - in any aspect of life.