| I and Thou
You do not need scientific studies to realize that humans are promiscuous. This is everywhere apparent. You do need scientist to observe in detail how and why this occurs and perhaps to reduce some of the hostility and suffering associated with normal primate sexual behavior. Baker and Oram have documented a host of reasons for accepting that humans are serial monogamists. They argue that both men are women are built to support multiple sex partners. [i]
For some, life-long monogamy is a successful path either because the couple is fortunate in their compatibility or because they have few alternatives and divorce is costly and painful.
There is no compelling reason to believe that life-long monogamy is a good idea for all and is not the way of nature. The emphasis in most primate groups is group affiliation and support and not the stability of a dyadic relationship. Human relationships are always evolving and changes are not always welcome. The reality of marriage is that each spouse continues to re-evaluate the marriage choice and innate tendencies to seek other mates will show up as sexual fantasy and as desire to improve the mate. Fantasies of other partners are common, if not universal, and will even occur during intercourse when you might expect total concentration on the mate close by.
Sexual dissatisfaction is common in marriages and a new partner may restore an impotent male or frigid female to full sexual capacity. Men remain interested in casual sex and women remain interested in rogue males and romantic interludes that offer genetic diversity. Awkward and plain young men may marry their high school girlfriend but twenty years later when they are successful, affluent and more confident, they may leave their aging wives for younger women. Successful middle-aged men want two or three young women to crew on their yacht and to share their bed. Aging men who desire younger women are disdained by their wives and women their own age. Aging women remain competitive and are understandably angry when their aging men prefer the smooth skin and shapely breasts and buttocks that only younger women possess.
Men, intellectually, can appreciate the deeper qualities of their female peers but are helpless to change the innate preference for young feminine beauty. The older man will refer to the "painful longing” he experiences when he sees a beautiful young woman. In that moment, the universe possesses no greater promise than the succulent body of the young woman. A sophisticated man will want more than sex – the young woman’s affection and devotion is part of his fantasy and may become real if he is wealthy and wise. Young women desire the wisdom, prestige and wealth of men who are older. Young women often benefit financially and socially from a liaison with an older man and may be better appreciated. They will also want a healthy, younger man with good genes to sire children.
We have recognized that jealousy is a cognitive-emotional complex that regulates sexual behavior. It is unwise to assume that a well-established monogamous relationship is stable and enduring. There is a continuous balancing operation going on in every mind that compares the benefit of every potential reward with the potential risk. Attraction to other people is normal and if the opportunity arises, intimacy and sexual encounters are rewards. The anger of a jealous mate is the risk.[i] Robin Baker and Elizabeth Oram.Baby Wars. Harper Collins 1998