I'd wager it's fairly hard to forget instinct as if you look at most animals when the are weakened or defenseless they tend to be very paranoid and reclusive in behaviour (ie molted animals, mammals hiding in boxes and burrows etc). Living creatures can and will adapt to whatever they feel is best though as humans can generally do things that are extremely unnatural (high rises, fireworks, fire, doctors, dentists) without problems.
Speaking from experience it seems to only appear in at a certain age, for a long time me and my friends (differing genders) were perfectly fine playing in the bath together naked with no qualms at all. The only incentive to remain clothed was my parents asking me too and that it was cold. Only until becoming a teen did that suddenly seem funky and really only because of sex ed. I suspect it is mostly just societal expectations honestly and the fact it is taboo similar to wearing a latex suit outdoors but without the anonymity.
Speaking from experience it seems to only appear in at a certain age, for a long time me and my friends (differing genders) were perfectly fine playing in the bath together naked with no qualms at all. The only incentive to remain clothed was my parents asking me too and that it was cold. Only until becoming a teen did that suddenly seem funky and really only because of sex ed. I suspect it is mostly just societal expectations honestly and the fact it is taboo similar to wearing a latex suit outdoors but without the anonymity.
