I am not sure that all boys pre-teenagers or teenagers liked to be naked in front of their mothers at a physical exam in front of a doctor -male or female - and sometimes with a female nurse present, it could mean that they were naked in front of three women but even if the doctor was male , it could be embarrassing if a boy was shy towards nudity, all were not exhibitionist or used to be seen naked at home by mothers
And I am not sure also that a doctor was conscious that a mother enjoyed to see her son naked and that a son enjoyed to be seen naked by his mother, no matter was his age even until last grades of high school or college, I believe rather that many doctors especially male ones were old school and did not mind about the feelings and thoughts of a boy, male nudity was for them pretty normal because it was not manly to be shy and modest, so if a boy was reluctant to strip completely, he might get used to it even in presene of his mother
And eventually, I am not sure that all mothers enjoyed to see their sons naked, the nudity was still taboo in many families, they were present because it was the social norm, a kind of implicit rule, that a good mother caring well about the health of her son came with him at his visit to a doctor and if the doctor required full nudity they saw their son naked but it was not by pleasure..
And as regard woman doctors, it could be for the same reasons than for a male doctor, a boy might not be shamed to be naked, may be also because they liked their power to instruct a boy to strip completely in front of mothers and possibly nurses when they were teenagers, a kind of CFMN situation that they liked.
As regards swimming in the nude for boys as a rule at school or YMCA centers, I doubt that all boys enjoyed to be seen naked by not only mothers and friends but also very often by sisters and friends or aunts or girls classmates, some enjoyed it certainly but others were mortified to expose completely their naked bodies to a female audience. Even if some boys stated in relations that they liked it , it might be the feeling of all boys.
what was the most interesting was that when there was a discussion at a schol board to decide of the maintain or not of the rule of swimming in the nude for boys, it is related that very often the mothers were the most numerous to vote in favor of the maintain of the rule but I think that it was more for the respect of a school tradition and for a rule who was supposed to be a good way to discipline strictly boys who were supposed to be unruly than because they liked to see their sons naked...