If you are interested in the design of the human sex organs, there are some fascinating theories suggesting how they may have evolved in order to perform some complex biological functions.
Scientific American recently recycled a theory about the design of the penis propounded by Gordon Gallup. (Secrets of the Phallus, April 2009. Gallup's theory had been published by the BBC and New Scientist several years ago: Penis is a competitive beast, August 2003.) According to this theory, the shape and thrusting action of the penis has the function of removing old semen (including that of rival lovers) before depositing a fresh load. The partial lost of erection after ejaculation is an important design feature; it makes sure that the new semen is not removed as the penis is withdrawn.
Meanwhile, a separate theory, the delightfully named "upsuck theory", explains how the female organs are designed to select the sperm from the most desirable lover, pumping it upwards towards the uterus during orgasm.
There are significant problems with both theories. See for example Harry, Sally and Evolutionary Biology, which suggests that if these evolutionary mechanisms really worked, then humans would by now have evolved to have mind-blowing sex all the time. See also my earlier post on Female Pleasure, and an interview with Elisabeth Lloyd in Thinking Meat.
But it is what happens when you put these two theories together that bothers me here. Both theories appear to rely on POSIWID thinking - looking at some biological feature and trying to infer its purpose (in terms of a positive contribution to the survival and reproductive success of its owner). But it is usually incorrect to think of a single biological feature in isolation. These bits of biology interact to form complex systems, frantically pumping live semen in all directions.
So what (if anything) does POSIWID tell us about the whole system and its purpose? As Žižek argues (in To Read Too Many Books Is Harmful), human sexuality is not limited to the biological sphere: "sexuality is the very terrain where humans detach themselves from nature ... a drive that gets thwarted as to its natural goal (reproduction) ... thereby explodes into an infinite, properly meta-physical, passion".
The POSIWID principle doesn't mean we have to reduce all explanations to unobservable biological mechanisms. Human behaviour is driven or constrained by many forces, including social and spiritual ones.
See also my post on Redesigning Sex.