Us prudish westerners are kind of accustomed to the notion that showing images of sexual activity or erogenous zones is more or less taboo, although we're usually comfortable with it if it's part of what we consider to be "art".
Displays of these parts of the anatomy seem to generate less concern in many other cultures.
I teach a course in human physiology, and one of the things I've been struck by is that while the students are somewhat uncomfortable and embarassed by the subject of sex and reproduction, they are much more uncomfortable about the gastrointestinal function. In fact, that seems to be the thing that embarasses them the most. Why? I have no idea.
But it occurs to me as I think about sexuality and reproductive matters in the arts of tribal peoples, that depictions of gastrointestinal function seems to be completely avoided in those cultures, too. I don't recall seeing a single item of tribal art from any part of the world that depicts defecation. Not in images of humans, not in images of livestock (where the product can have economic importance).
I have nothing profound to say about it, but I think the apparent universality of this taboo is interesting.
There has been some comment in other arenas about some of the taboos you mention.
The attitudes about the gastrointestinal processes are likely linked to those for sex in part because the organs of excretion either are or can be used in sexual behavior. And of course, the gastrointestinal products are not appealing on several scores, although again traditional societies seem often not to share our degree of aversion to them since animal feces were/are? matter-of-factly gathered by hand and used as fuel in many of them.
I think some of the attitudes and taboos about gastrointestinal processes and products are sourced in the basic logic of not taking back into the body any of its excretions.
William Connolly, a political and moral philosopher, has as the arresting title for one chapter on such taboos, "Don't Pick Your Nose and Eat It!" That gets one's attention. And quickly triggers disgust.
And Erving Goffman in one of his books provides a little experiment that permits one to map this taboo personally. He says, work your mouth to increase the amount of saliva in it, as you might as you were preparing, say to wet some thread to thread a needle. Notice that there is nothing disgusting about this saliva as it moves about in your mouth. Now, he says, envision that you have gradually filled a drinking glass with your own saliva. Think how difficult you would find it to drink that glass of liquid.
There are some Indian health faddists who do claim that drinking one's own urine has sanguine effects but most of us remain unconvinced to the point that I suspect none of us is tempted to try it.
Interesting stuff though.
R. John Howe