I am surprised to learn that the other primates have such short penis lengths as compared to human males. My sister visited Africa a few years back and returned with the most amazing picture of some sort of monkey sitting with it’s penis erect, which appeared quite long, in fact long enough that it was giving itself a blow job!
Could the large human penis length be related to the fact that unlike the other primates, humans walk upright?
Exactly! That and the “willing to put in” part. 
Indirectly, yes, based on the theory that penis size matches the female birth canal. Bipedal locomotion drastically altered the birth canal (babies facing posteriourly in humans while in most apes they face anteriourly at birth, longer more difficult birth and higher mortality rates than compared to other primates, humans require assistance during birth while most primate species deliver by themselves, and may other costs of bipedalism), therefore requiring the adaption of the penis as well.
I recall reading somewhere (Science of Discworld?) that chimps, especially bonobos, are as horny as humans, i.e. they have no special period when they’re in heat but will pretty much go at it, whenever there’s an opportunity. Seing that they are the closest in average size to humans, this might imply something. But I’m not sure what.
Not Darwinianism since that says a species will fill a void to take advantage of an available food supply.
However it does have implications in the selection of a mate which affects the species which doesn’t have to worry about food supply. The selection of the mate is also affected by medical (plastic) and dental science.
I am not sure about the size issue as it relates to Darwinism, but I am pretty sure my prehensile penis will be of great benefit to my descendants.
Fuck darwin!
Wasn’t he the Asshole who said Africans are “more ape like” and all that shit. Well if they “are” more ape like, then they as a whole should have the little dicks.
As far as the above quote goes, Why do you classify yourself as a primate?
Maybe because humans are classified as primates? 
Bonobos have quite the sex life. When I come back, I want to be a bonobo. Sex is actually the fabric that keeps bonobo societies functional. A good account of the bonobo’s sex life can be found here, which is an old SciAm article.
acsenray: While humans do have a comparatively large penis size, chimpanzees are considerably larger in testicle size. This is because they have many sexual partners in a short period of time, so the amount of sperm discharged becomes critical. If pumping dislodges competing sperm then this would most likely be what caused the adaptation of the penis when the birth canal changed, as CairaJade said.
And thanks, Epimetheus and CaptBushido. 
So it seems we have an “arms” race of sorts…mother nature creating larger birth canals and males developing larger peni and testes…where will the madness end?
Well, no. The arms race I think you have in mind is like in sexual selection where one sex develops a sexual marker and the other sex an affinity for it. This is a self-feeding mechanism, and some examples might be long tailfeathers, red bottoms etc… In the penis example, the extended length/girth of the penis is necessitated by the change in the birth canal, but the change in the birth canal is caused by the increased cranial capacity (or something to that effect, the exact cause is irrelevant) so this is not a self-feeding mechanism. Due to our comparatively monogamous nature, testes aren’t about to have a big increase either.
Not sure where your definition of Darwinism is from. The more common definition is “a theory of organic evolution claiming that new species arise and are perpetuated by natural selection”. However Darwin was probably more interested in sexual selection which was mentioned before, so that can also be called Darwinism
Diamond discusses the issue at greater, umm, length at the, umm, end of his Why Is Sex Fun? (Weidenfeld & Nicolson, 1997).
Basically, he regards the issue as unresolved, though he considers most of the theories raised in this thread. He suggests that, because of the limitation of the average women’s vagina, there is a practical upper limit to the size of the male member. New Guinean phallocarps are then an example of what human penii would look like if there weren’t this constraint. But he suggests that the primary audience for penis size are indeed other males. Yet he qualifies this thought in various ways. The final sentence of the book is therefore: