Penguinness said:
Yet sunbathing is fully acceptable.
EXACTLY! I find it odd that swimwear and underwear are basically equal as far as coverage, yet they are very different in their social acceptance and level of "decency." Suppose someone just got back from swimming in a bikini and then put some shorts on. As she's walking down the street, she may get some glances, but people might generally think, "Huh? Oh, that's a bikini top. She must have been swimming earlier, so it's ok." But then suppose another day, she just didn't feel like wearing a shirt. So they walk around in shorts and a bra (showing exactly the same skin as before). But this time people would probably think, "WTF? Is she wearing just a bra? Put a shirt on lady!" Anyway I think it's an interesting phenomenon. Also, a person's lack of clothes being deemed "indecent" can be very arbitrary, varying depending on who the person is that's wearing(or not wearing) the clothes. "Is she hot?" Is a great indicator of this. Babies and hot people are accepted for wearing less clothing, where as fat/ugly/elderly people are shunned for wearing less than a parka.
I think some insight can be gained in observing how this relates to young children. Very young children have a tendency to be somewhat averse to clothing. I know someone who's kids will take their clothes off any chance they get. In children's minds, being naked seems to be the optimal state. In the majority of the home videos of me and my siblings as young children, we were half naked about half the time. Our typical outfit would be a diaper/undies and sometimes maybe a t-shirt. Even my sister would go around in just a diaper. Some videos, she would start out in a dress, and then a minute later she would pull that thing off. Anyway, my point is that children are trained to be clothed. They have to learn it. They are taught that nudity is bad and indecent. The human body is indecent. No wonder our culture has issues with body image problems.