@xris,
While I don't have the luxury of time to go into this discussion at any great length, I wish simply to put out my take;
as just that . . . one take. (I don't really intend to defend it)
I tend to think (and I have looked into this matter a little bit) that one major impetus for this development, had been the tendency from a number of sources, in the development of social meta-grouping, to remove the human being from the pure part of nature that it is, and put this species on a plane
above and
detached from, all other members of the planet earth. This is something which underlies religious belief-systems' propagating sexual taboos even further--
in most cases (think 'temple prostitution,' 'Kama Sutra,' etc. as out of the common norm)
We can argue that there is fair and healthy porn (which genre is a useful tool for long-term relationship bonding), and damaging and unhealthy porn (not just visual media). I tend to think that most porn is of the second genre; and it is addictive (although many other things are too, actually). Also as Aedes has pointed out, it can be damaging to sexual function (there are some theories on just what is happening, but I can't recall the details at the moment).
However, sexuality is a very, very important element of the human condition, both physically and neurologically, and a greater degree of honesty and truth about that is very late in coming in many parts of the world. One good book written by a female professor here in Japan dealt with sexual help for certain handicapped and invalid type patients--
an actually very psychologically important point often overlooked by healthcare institutions, I think (I could be lacking in information here).
Being nude on the beach should be little different from being nude in the locker room, or in the open air hot spring--
although there would be social etiquettes that would surely apply in each particular setting. However, few cultures are accustomed to the more natural mind set. This is simply nature. (I recall the woman's underwear designed for those of 'asexual' orientation which frankly had written on it, "
Get over it; it's only underwear.") It also reminds me of the comment of one Japanese artist whose sculptures for a certain park were all nudes. When asked why all the figures were nude, he replied something along the lines of 'in nudity, we humans are all equal.'
The message I always try to get at, is that we H. sapiens are just as much a natural part of nature as any other, and all other, parts of nature, and in that way, should be a little more honest about it--it's healthier (in some ways, to a degree).