Just curious because I can’t think of any other animal that even considers this. Or maybe there is; I don’t know.
So is this pretty much a learned social behaviour or what?
Anyway, I know the jokesters on this board are probably foaming at the mouth to get their one liners in; which is fine I guess. (hell I like a good laugh) I would however like some serious “input” (snickers) as well.
In case anyone was wondering this thread was inspired by some recent (trite) threads going on in IMHO.
As far as animals go, penis size is irrelevant as usually they are hard to see and a big penis has no advantage and possibly disadvantages- other sexual markers (e.g. a bright red bottom) are used.
As far as humans go, it is a cultural / joke / sexual thing that most people find humerous and occassionally slightly threatening. I am not quite sure what the question is?
Well, for humans, by the time the female sees the male’s penis, they’ve pretty much decided to sleep with them anyway.
It’s clearly learned social behavior. There is a completely different attitude toward penis size now then there was whan I was a teen in the 60s.
However, the chance of pregnancy increases as intercourse is repeated - which may(or may not) be more likely to happen if she likes what she sees.
Also, a guy with a larger penis may sleep around more. More confidence and girls tend to share this information with their friends which may lead to the guy getting more tail. Tommy Lee comes to mind.
I may or may not be speaking from experience. 
So is the human penis larger, in proportion to the rest of the body, than most other mammals?
I believe that the flea has the largest penis in relation to its body size of all living creatures. Thus explaining the popularity of Red Hot Chili Peppers.
I have heard that as far as animals are concerned the marine iguana has the largest penis in proportion to it’s body. 
Compared to other primates the human penis is proportionally much larger. Average lengths in primates are roughly as follows:
1 1/4 inches: Gorilla
1 1/2 inches: Orang Utan
3 inches: Chimpanzee
5 inches: Man
AFAIK, an adaptionist cause is yet to be found for why humans have such large penises. One theory that is obviously popular amongst (male) anthropologists is that penis size underwent sexual selection in our lineage. While one might appeal that once one sees a man’s penis sex is pretty much assured, surprisingly enough clothes are absent in most of our geological record, so it may stand to reason that females did in fact previously see penii in the broad light of day. Critics of this notion tend to appeal that penis size isn’t very high up on Jane Doe’s shopping list for Mr. Right. However, given the incredible diversification of modern human sexuality which defies adaptionist reasoning, it would seem to me that it’s possible a mild subconscious urge which has caused this obscene elongation of man’s favourite organ may be all but undetectable to our modern whimsies.
There are a million and one variations to the standard positions on this, but it would seem a sure bet that penis size does have some darwinian explanation. Some might cite non-adaptive reasoning as a cause (genetic drift etc.), but I can’t really believe that we’ve become the Ron Jeremy of the primates without a solid cause. Few are willing to put in the long, hard hours to research in the field of comparative penis anatomy, but if any budding biologists are looking for a field to excel in I’m sure their input would be greatly appreciated in this virginal body of science.
The fact the the humans penis is a bit larger than is needed (and much larger than a gorillas - take that you stupid monkey), is perhaps a sign that is evolutionary speaking a sexual marker. The actual anatomical size cannot be “cultural”, (unless pulling to much on it is what makes it as big as it is), which is why I was puzzled at the question. If by cultural the OP means “women select for big penis sizes, as opposed to the penis being as big as it is because of some strict anatomical reason” then the answer is probably yes. This is still evolution, but comes under the term “sexual selection”. However, culture usually refers to that bit that is not directly part of our genes, but learnt.
I remember a Discover article from about 10-15 years ago that suggested that the issue wasn’t so much female response to big 'uns but other males’ response thereto: “Oh, man, I don’t stand a chance, better back off.”
Any credence to this theory? To me it strikes of overextrapolating from elk or moose or something. I mean, it’s not like guys actually, um, swordfight.
Maybe they did. there was not much else to do on those long evenings in the cave
I hope this is right, that makes me above average.
The penis can act as a pump in the female vagina, sucking old sperm out so that that mans sperm has less competetion. Maybe a larger diameter penis makes for a better pump. I dont have a cite for this it comes from a study i read a year or so ago that talked about sex and cheating and how after a wife has been cheating and/or gone a while men instinually have sex in a fashion that sucks other men’s sperm out. Better safe than sorry.
As far as length, you’d assume a longer penis would have a better chance of shooting sperm directly into the cervix. a 3 inch penis may just shoot sperm around the cervix while a longer one would have a better chance of getting past.
I was just reading about this topic in Jared Diamond’s book, The Third Chimpanzee: The Evolution and Future of the Human Animal. (This is the same author who wrote the Pulitzer Prize-winning Guns, Germs, and Steel, although Third Chimpanzee pre-dates the other book.)
The book included a figure entitled “How females see males,” and compared chimpanzees, gorillas, humans, and some other ape. Normalizing for differing body sizes, humans have the longest penises, and chimpanzees have the largest testicles.
Anyway, IIRC, Diamond purported that large penises in humans are indeed primarily intended to impress/discourage other men. His evidence for this included women’s oft-reported lack of interest in penis size contrasted with men’s extreme preoccupation with the matter.
He also discussed what he thought human penises might look like if not limited by the size of women’s vaginas: specifically, the practice of New Guinea natives to construct two-foot long, 4-inch thick decorated penis sheaths, topped off with decorative fur on the end!
If I remember correctly what I read in Robin Baker, the human male has proportionately the largest penis and testicles among mammals. Humans also have the longest sex act with lots of in and out, which is generally not true of other primates. Baker sees it as a manifestation of the critical role of sperm competition in human evolution (which means that generally speaking, human women are very likely to be carrying around sperm from more than one donor). Largeness and pumping play roles in overwhelming and dislodging competing sperm.
IIRC (from one of these very SDMB threads), human male penis length correlates pretty well to human female birth canal width–which is the largest of any primate (to acommodate our huge heads at birth). I believe it was mentioned that the unusual penis length was necessary to ensure reasonably good suction in the wide birth canal.
If this is deliberate, you are a genius:
(italics mine)
Classic, truely classic.
What do you mean? It’s all italicized.
Yeah, my fault in coding, I should have bolded the parts I meant, or got rid of the code that made the whole post italicized, lol.
I think these are the things he was praising (a few may be me examining the paragraph just a little too hard, though!), and if it is intentional, then I, too, would like to give you the proverbial gold star!