kelley:
> um, oh my, people--men and women both--actually like anal sex, yes? you
> know, of course, that women have anal sex, too, yes? and what is bad about
> semen on the face or any other body part for that matter? does that fact
> mean that a woman didn't come also?
> ...
But this is not what's going on in pornography. Pornography is depiction, so the real action is happening in the living room or the movie theater where the customer witnesses the depiction. Whether the participants enjoyed anal sex or various ejaculatory trajectories doesn't matter much -- they've long since gotten paid and gone home. In any case, just as in other kinds of art, many appearances are actually simulation and illusion, and most of the customers are well aware of this. It is _observation_ that matters.
I don't have an answer as to exactly what's going on, but I've read and observed directly that at least some primates and other reasonably intelligent animals practice sex in social groups -- not only publicly but with some kind of audience participation. (Some humans do this, too.) Also, I've read that chimpanzees (or some other advanced primate) can't function sexually unless they've observed sexual behavior when young -- they don't know how to do it and have a hard time figuring it out.
Suppose humans are like this; then, simply as a matter of survival, they would have a need and desire to observe sexual behavior and a concomitant desire to be observed, at least some of the time. But when slavery was invented, especially the slavery of females as a class, then the dominant males would have an interest in not only locking up and hiding the women (who had become property) but also hiding and effacing sexual behavior. Later, when trade and capitalism came along, they would find the scarcity created by this hiding and effacement and being selling into it. Shame-facedly, deprecated and even persecuted by the noble dominant males -- but there's money to be made.