When I was growing up the schools in my town did not have swimming pools. We boys were sent to swimming lessons at the YMCA, beginning in about 1952. The swimming and locker rooms at the YMCA were all naked. Even in my otherwise prudish family we grew up understanding that the only problem about nudity was if a girl or woman saw us naked. Being seen by boys or men didn't matter since all boys and men look the same. I didn't know any boys who were concerned about being seen by other boys or men. We had never heard about gay men in those days.
At the YMCA it was also common for men to exercise naked in their Gymnasium until around 1955. I also went to YMCA CAMP in the summers starting around 1952 or 1953. At camp we always swam naked in the lake. The camp had a Camp nurse, and occasional parent visits, but she didn't go to the swim area during swim hours. Otherwise was all boys and men.
I began middle school in 1957. At middle school we wore shorts and a T shirt for PE class which was in spaces shared by girls. The locker rooms had a large common shower area where we were all expected to shower together. Again, I had no idea that it would be a concern. I remember my mother talking once about girls having to shower together in the girls PE class. She said girls were more concerned about being naked so it didn't matter for boys but girls needed more privacy. And so it began.
One big part of the cultural revolution going on in the 1950s was the move from small (walking distance) city homes to larger separate homes in the suburbs. Having a large family in a small home prevented individual privacy. Before the 1950s kids had grown up seeing naked adults and being seen naked by adults in their family. Beginning with the automobile culture in the 1930s, and BOOMING in the 1950s America moved to private homes in the suburbs. Everybody had PRIVATE indoor bathrooms, and private bedrooms for parents and children. Families became smaller after the 1960s too. A family with 1 to 3 children can be a lot more private at home than a family with 5 to 8 or more children. "The Pill" was distributed in 1961. I grew up in a privacy home with a mother and two sisters, but I never saw a naked girl anywhere until I was 18. Before the suburban revolution of the 1950s, most people did not have the physical space to be private in their homes. Boys especially grew up seeing other boys and men as NORMAL every day at home experience. People didn't see a need to make it different at a YMCA or boys swim.
Then also the culture began a "Sexual Revolution" in the 1960s. While I grew up in the 1950s I did not know that homosexuality or homosexuals existed. I did not know any friends who had been molested. Sex between men and women was a topic they didn't teach boys until high school, and only taught girls younger because they started bleeding. Nudist magazines from the 1950s all featured lots of children photos along with mostly naked women. I found a stash of HE and other nudist magazines in the summer when I was 18. I was amazed and confused. I wondered how anyone, and especially women, could just be casual about being naked. And there they were photographed in these magazines. I hadn't known that Nudism was a thing or that nudist camps existed before finding those magazines. We all swam naked, but we weren't nudists, and only with boys. Here were men and women, girls and boys together. What was I to think. Later that summer I also saw an actual naked girl, up close and personal. I was growing up.
In fall of that year, 1963, I went off to the big city, Seattle, to University. In 1965 my roommate told me that the University Drama Dept. had a lot of queers. He had to explain what that meant. I'm sure that was part of the cultural changes which now prevent nude swimming at the YMCA and elsewhere.
The linked videos conclude by saying that nude swimming will never return. Never is a very long time. Cultures change. It may come back.
