But this same-sex prudery is a new phenomena. I can't speak for girls, but for generations boys swam naked at YMCAs and summer camps. Gym showers were a required part of hygiene. Nudity was routine in military barracks and college dorms.
All this same-sex prudery is something that has come about in just the last 20-30 years. I think the biggest factor is this change is the openness of homosexuality now. It makes same-sex nudity take on a sexual connotation that didn't exist when I was a kid.