There is one mystery that continues to evade scientists of numerous disciplines like “archaeology, anthropology, and zoology, as well as the evolutionary, psychological, and sociological branches of biology”: “why, comparatively, man’s penis is so disproportionately large” [1,2]?
It is true that “virtually all human penises are big in comparison with those of the other 192 primate species. Flaccid, the penis of the gorilla and the orangutan, both with much bigger bodies, is virtually invisible; erect, it reaches 1.5 inches or less; the chimpanzee, man’s closest relative (sharing 98 percent of his DNA) achieves an erection twice that of the other two apes but still only one-half the average human one” . It seems obvious that this human superiority cannot be explained solely by the requirements of sexual reproduction, since “the male ape successfully propagates his kind with much less”. So another theory is put forward that “the human penis has also become an organ of display, like a peacock’s tail or a lion’s mane” .
Jared Diamond states in  that “the human penis is an organ of display … intended not for women but for fellow men” and the main role of this organ is to pose a threat or emphasize a status. But, I think, there is one problem with this explanation. Not only human males are generously endowed by the nature. “Human females are unique in their breasts, which are considerably larger than those of apes even before the first pregnancy” . The simple rough charts below demonstrate that men and women show the same tendency in exceeding their ape counterparts.
The real explanation must be applicable to both genders. It is highly unlikely that women’s breasts are organs of display for “fellow women”. It is probable that men’s and women’s organs are indeed intended to be displayed, that is why, their sizes are so noticeable. Also it is notable that here we are dealing with primary and secondary sex characteristic, which allow to distinguish with certainty one gender from another. So, in my opinion, the display in both cases is addressed to both genders and it has some social function. There might be definite scientific grounds for nudist practices not to cover the body with clothes from time to time, since man and woman have evolved to be nude and not afraid to show it.
1. Thomas Hickman, God’s Doodle: The Life and Times of the Penis, Soft Skull Press, Berkeley, 2013.
2. Thomas Hickman, Slate: Average penis size
3. Jared Diamond, The rise and fall of the third chimpanzee, Vintage, London, 2002.