Shouldn't we assume that every normal man of average built and of good health will also be equipped with a sufficiently large and fully functional penis?

Well, I do believe that there is sound biological logic why we shouldn't be so sure about this. Humans obviously have a biological character and a cultural character. Our cultural character accounts for much of our conscious behavior. On the other hand, our emotions and unconscious states of mind (including desires) are largely an affair of our biological character.

Our cultural character has changed tremendously over the last 2000 to 3000 years, but our biological character is still basically the same as it was for our stone-age ancestors some 10,000 years ago, or their predecessors some 50,000 years ago.

And there is no doubt that in earlier phases of our history, our sexual function and behavior was largely one of mammalian biology.

A salient aspect of mammalian sexuality is what Darwin defined as male competition and female choice. Mammalian males typically strive to inseminate as many females of the species as they can handle. Females display a profound preference for men with what they (unconsciously, in accordance with their biological character) consider the best genes.

But this is not yet the full story. Human females, as they developed throughout the past 6 million years, have more differentiated interests than just to mate with the male with the best genes. Human females have a material interest in male-female bonding. In order to preserve their genes, human females spend many years raising their young. And unlike other mammalian species, human females had and have to take care of offspring born years apart, and possibly of different fathers. (Great apes have an inferior procreative strategy, as females only give birth every few years, after having raised the offspring born in one year.)

Human and pre-human females, during millions of years of evolution, have developed a survival strategy that includes the permanent binding of a male partner. The benefit for the male partner lay and lies in the permanent availability of a female for sexual intercourse.

So far, this is standard anthropology.

But there is a twist, which is the definite interest of both the male and the female partner in being unfaithful.

For the male, the strategic benefit of unfaithfulness is obvious. Getting a shot at another female means an increased chance to preserve one's genes. To spread one's genes in acts of unfaithfulness, furthermore, is a low-risk behavior for men. There is potentially no additional obligation. The female is inseminated, and after that, she alone cares for the common pool of genes, or she enlists the help of another man who happens to be that female's permanent partner.

For the female, the benefit of unfaithfulness is subtler. In a human or hominid society where one-on-one bonding is the standard pattern, only a few women end up with the best males, while other females form permanent partnerships with less than ideal males who nevertheless can be of valuable help in raising offspring.

However, the biological interest of these females still is to mix their own genes not with the genes of lesser males, but with those of the fittest men.

The solution is female unfaithfulness: have occasional, even secretive, intercourse with an alpha male, and after being inseminated, take any male you can get as a permanent partner to help in raising one's offspring with an alpha male!

This is, of course, a complex pattern of biological sexual behavior. It's never a clear-cut scheme. Rather, the pattern is one of a trend. Because the female can bind a lesser male only by giving him sexual access, and by allowing him at least the illusion that all the offspring in the liaison are his, it will have occurred on a regular basis that indeed, a female human gave birth to the offspring of lesser males. A standard setting may have been that some of the offspring resulted from unfaithfulness with alpha males, and others from a permanent relationship with a lesser partner.

The whole thing developed like a kind of involuntary horse-trading.

Much also depended on the economics of the times. When economic basics allowed more complex societies to develop, alpha males were capable of maintaining a larger pool of females who were not bonding at all with lesser males. This is evident in the harem building of the alpha males of all early complex human societies. But harem building is an aspect primarily of male sexuality in situations where one man has to his disposal the means to exclude other males from a pool of females. Harem building is also an aspect of male dominance.

On the part of the females, living in a harem is probably much less in congruence with their biological interests as they cannot follow the trait of their biological character, which would be to monopolize a man.

Biological sexual behavior is obviously regulated through sexual desires.

Again, for men, this is rather straight-forward. Because the most promising sexual strategy for men is to spread their genes, and sperm, throughout as many females as possible, the behavioral sexual goals of men is to either possess exclusively as many females as possible, or to inseminate not only the one or few females he directly possesses but also, in acts of unfaithfulness, as many females as possible who are in permanent bonds with other males.

Therefore, male sexual strategy definitely is directed at a multitude of females. And as the regulatory force is desire, it cannot surprise that there is a clear incentive to add new partners: sex with new partners typically is more exciting, so there is a higher likelihood of orgasm and ejaculation.

For the female, the most promising sexual strategy is to first play it safe: have one sexual partner in a permanent relationship who can provide material help in raising the offspring he considers his own. However, beyond this, a success-driven female strategy includes to seek better, or the best, genes to pair with through regular acts of unfaithfulness with alpha males.

For such a setting, female sexual desires have to be more differentiated than those of men. Nature regulates this by not allowing them full sexual satisfaction with the lesser male who may be their permanent partner. They may bond ok with a lesser male, and develop the emotions summarized as "love", but there is an element that becomes obvious to them only over time, and usually only after having given birth to an offspring fathered by a lesser male (thus already making sure of her genes' survival in principle). This element is her desire for sexual satisfaction with a male who is better equipped to bring her to the level of sexual pleasure that results in a vaginal orgasm. Thus, female sexual, more than male sexual desire, is dualistic: have a love relationship with a permanent partner, and seek sexual satisfaction with an alpha male.

If biological science teaches us something than it is this: nothing is accidental. Every phenomenon we encounter in the world of living species has developed through evolution and natural selection in precisely the fashion we encounter it because this is what has proven to be the best survival strategy.

For this reason we have to make sense of the following: almost all men have no problem, and great pleasure, in achieving orgasm and ejaculation through sexual intercourse with a female. But females only achieve sexual satisfaction through vaginal intercourse with men of an alpha built (which means: men who have a particular large penis). The above explanation tries to make sense of this phenomenon by assuming that the female incapacity for vaginal orgasms with men with penises of an average size has been developed in the course of evolution in order to make sure that the females of the human species will seek out sexual relationships with alpha males (large penises) in spite of being bonded to lesser males (average penises).

Based on the above anthropological considerations, I believe that penis size is a valid genuine concern for all men. It's not a matter of locker-room vanity. Only men with large penises are well equipped to bring women to penile-vaginal orgasms. And men who can provide this to their female partners will not have to fear the humiliation of their permanent partner seeking out better-endowed alpha males. Apart from that, alpha males stand a good chance to be sought for encounters of unfaithfulness by females who are bound in permanent relationships with average males, simply for their likely capability to enable women to experience the superiority of the vaginal orgasm.