Evolution and Size
I saw an interesting program a couple months ago on Discovery Health about Sex and Evolution.
Apparently the vaginas of female monkeys are located where the anus is located for human females. This, and the fact they walk on all fours, is why monkeys always do it doggie style. When female monkeys are in heat her vaginal lips get red and swollen and can be easily seen on her backside as she walks on all fours. This lets the males get turned on and want to mate with her.
As humans evolved from monkeys they began to walk upright on two legs and the human female vagina began to evolve towards the front of her body for frontal intercourse. Now that the vagina was not in view (while walking upright) women started coloring their mouth lips with lipstick to attract males (it resembles the swollen vagina lips) and draws a similar response.
Another side effect from her vagina moving to the front was that the male penis needed to be larger then before to penetrate her in a way that it could deposit the sperm close to her egg. The show stated that over the course of millions of years males with smaller penises had far less opportunities to impregnate women, as the women rejected them. Over time endowed men were allowed to pass their genes to the females and their offspring, carrying their genes, were better endowed. Smaller men died off.
Because of this favoring of larger penises by women the human penis, today, is proportionally larger than any other species (with regards to the overall size of the animal). Chimps penises are tiny. In comparison, human males are very large at a 5” - 6” average length.
I read somewhere else that with species where the females often cheat, the males tend to have larger penises. The species where females don’t cheat results in micro penises in the males. This was because men with larger penises tend to have a greater chance of impregnating a woman than smaller men. The longer penis ejaculates closer to the ovaries thereby increasing the chances that the sperm will survive it’s shorter journey to the egg. Also, when a woman has a deep vaginal orgasm which tends to be a contraction-type of orgasm this actually sucks the sperm towards the womb, and since larger penises are better suited to bring a woman to vaginal orgasm ..
As men (in these cheating species) are competing with other men who are screwing the same female, the sperm are actually competing to see who impregnates the egg. The sperm of the larger sized man has a much better chance of reaching the egg than the sperm by smaller men due to the placement of the ejaculation and the vaginal orgasm likelihood when woman is with large guy.
These are both evolutionary theories, but I found them to be interesting.