It's all about original sin. Whether you Adam and Eve it or not, ever since, fig leaves have been de rigueur. It is mandated to hide the crucial aspect of one's social identity. And yet it is also mandated to display it in coded form. You must not see my penis, but you must see my top-hat and tails.
The reason Sub-Saharan Africans were enslaved and Arabs, Indians, etc were not, is the shameless nakedness of the latter 'proved' that they were less than human. we should not pretend that we are beyond such things, when the whole organisation of society right down to toilet facilities rests on such primitive notions.
There is a natural disgust for human waste - cattle also avoid eating around their own droppings. this is extended to menstrual fluids, and becomes gendered such that self-disgust and its projection onto the world is a particular female proclivity. See also for example,
trypophobia.
The interweaving of instinct, social conditioning, and individual variability all contribute to the establishment of personal identity. We are coyly pretending that we are talking generally about all kinds of identity, but we all know somewhere, that what matters in any encounter with otherness, is to accurately identify the range of appropriate responses. Here you come, and shall I run from you, fight you, fuck you or eat you? Or some combination of these? Knowing the difference between a legitimate MD and a quack, or between a policeman and a postman is also potentially useful, but a minor, secondary question.
There are primal fears, and fundamental taboos in play in this discussion. The careful exposure of these to the insight of all participants is the prerequisite for anything approaching a rational or philosophical analysis.
Failing that, we are, alas, reduced to mere politics.