Humans, like many other terrestrial life forms, reproduce sexually. We, like all other sexual creatures, are subject to instinctive sexual desire triggered by appropriate criteria.
However, humans are unique in two ways. The first I mentioned in the discussion in Chapter Two Reproduction -- their anatomy has made sex more difficult.
It's the second unique thing about humans that makes their reproductive life unusual: humans can think. Thus, the criteria for desire and selection are greatly complicated. People apply not only physical, but societal, cultural and economic criteria to desire and selection.
The evolution of the human body and mind has resulted in an incredibly complex psychophysiology. This sets humans apart from how all other animals approach reproduction. Males compete for breeding rights, females select the best available male. Many female mammals come into heat, a limited period when she is impregnable. Before and during this period, physiological changes occur that are detectable by the male. She becomes the most desirable female around, and she wants sex. The males line up for her, compete for her, and she selects and mates with the best. When a mare comes into heat, she mates with the alpha stallion (the one that wins the mating battles). She doesn't think about it, she doesn't examine his physique or bank account; if he is the alpha stallion, he is the one with which to mate, since he has proven himself superior to other males. If she doesn't wish to mate with him, she simply walks away.
For other animals, instead of walking away, the female expresses her lack of desire by swatting the male. For example, a lioness, well equipped with weapons and close to the same size, can discourage any male by beating the hell out of him. He, having other females in his harem, shrugs his figurative shoulders and goes elsewhere.
Such is not the case for a human. Men rarely battle each other for breeding rights. Women don't come into heat: they can mate at any time, she can get pregnant any month, deliver any day. Women don't automatically mate with a man because he won a fight. However, people still apply criteria in selecting a mate, and those criteria are gender-linked.
The human male has a drive to impregnate as many females as possible, to create as many offspring with his genes as possible. (Ehrlichman & Eichenstein, 1992) Thus, he applies criteria typical for a male animal. He looks for women who are impregnable: those who are old enough to be past puberty, but young enough to care for children for at least several years. He looks for healthy (i.e., clear, smooth skin, "bright" eyes, good conformation of body and limbs, etc.) women, so they can carry the fetus to term, deliver it, and care for it after birth. Beyond that, he doesn't really care. She doesn't have to be intelligent, talented, socially aware, or in any other way have a brain. In fact, the dumber she is the easier it would be for him to meet her criteria for desirability since they are less likely to be extensive.
Thus, men have minimal criteria for sexual desire; basically, they are concerned with a woman's anatomy -- as long as a woman looks young enough and healthy, she is desirable. They also consider her beautiful, since to a male beautiful and desirable are virtually synonymous.
What is considered healthy-looking has varied over the years and centuries, and from culture to culture. In periods when there were food shortages, a woman that is now considered obese was thought attractive since her appearance clearly showed she had ample reserves. Other changes such as cosmetics to produce a healthy appearance, costumes that exaggerated the hips and thus gave an impression of an excellent child-bearing structure, etc., have increased men's perception of a woman's desirability as a sexual partner. Of course, few men consciously relate certain features with health, and thus that is why they find them attractive. They simply find women with such features sexually attractive, and that's enough without analyzing why.
Many characteristics are deemed attractive by the culture. That is, they are learned. The human male has a mind as well, and is taught much of the way he is supposed to regard the world. This includes what the female features are that he should consider attractive (i.e., sexually desirable), including non-physical as well as physical attributes. Such non-physical attributes include a woman's mind, accomplishments, and prospects.
Nonetheless, although his culture and society may tell him that he should consider more than anatomy, "people are likely to express approval for socially approved characteristics rather than for what actually attracts them." (Daly, 1983, p. 304) Deep down inside he still howls at the moon when a woman meeting his physical criteria walks by. For a man, thinking reduces sexual desire ("think about baseball").
This does not mean that the human male is a walking hormone. He, like the female, is a member of the human race, and thus is also aware of human society, its constraints and demands.