Advocates of polyamory and polygamy always make the erroneous analogy between sex and friendship, and between sexual and asexual relationships in general, although either is different by nature. Sexual love cannot be divided among several partners, the way our love for our friends, children, siblings ... can. There is a scientific reason for this: the chemistry of sex, as observed in most mammals' behavior and examined by neuro-scientific methods of brain scanning, shows that humans and other mammals always have a preference for new & hard-to-obtain sex partners, chasing whom releases the highest dopamine levels.
This makes impossible for any group of people, sexually involved with one another, to be fair to each one within that group. It's not enough then that somebody is sexy, smart, loving, caring ... to attract us; they must also be difficult to find, and different from those we find familiar. No matter how hard they try to appeal to us, being kind, gentle, helpful, and seductive, we will find a new partner always sexier.
It's the chemistry of human brain that is responsible for our acting in such a heartless manner. The dopamine levels released in our brain go high at the onset of every new affair, then gradually fade. It's the hormone connected with addiction and compulsive behavior too. When our primitive limbic brain is constantly stimulated by that hormone, our higher brain becomes paralyzed, unable to operate its reasoning skills.
In a so-called open relationship, it becomes extremely competitive, nearly impossible, to keep somebody's love: to be certain they love you, or not. Once someone else they like shows up, you automatically become "not new." Even if you accept it, being second, third, or fourth on their list, you still have to work hard to stay in the competition and not be kicked out: changing your looks everyday, talking and acting differently, showing your hidden skills and using up your whole reservoir of resources, till you eventually succeed in looking like a "different" person to them, so that they "love you at first sight" each time they set eyes on you.
If you do none of the above, they won't be interested in you. Still, the very fact that you show your love, care and sympathy for them makes you an easy-to-get "hunt"; hence they find you unattractive either.
This is how much effort we need to exert in order to live in such kind of relationships, and keep up with our promiscuous butterfly nature, for us not to be disqualified from that feverish, mindless, lifelong race. Those relationships consume much energy and time, that even a lifetime is not enough to devote to. Again, it's doubtful that in the end you will succeed in maintaining the love of each of your sweethearts, because "different and hard-to-obtain partners" will always be the prime targets Nature lures us to aim at.
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When a group of people agree on living in a close-ended polyamory or closed love group (a less liberal form of promiscuity, where no affairs are permitted outside the group), it's still difficult to know who loves whom and how much, within that group.
For instance, in a lover-group of five people (like those in science fiction books), no one can love the other the same:
Nobody feels special to anybody, nobody is bonded to anybody, and nobody can predict the moment when they eventually stop being sexy to the other, even when they gain a new status, from lover to friend:
Sometimes people in open relationships keep their old partners not for sex, but for the friendship, familiarity and attachment that develop between them. They become interested in them as good friends or even family members, but not to have or prefer sex with.
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It's ironic how the chase/pattern is repeated throughout the ages and across species, casting doubt on man's highly praised free will and love's superior motives, while it all comes down to chemistry:
It's our animal nature that urges us to chase every new partner to spread our genes and have more, better offspring. The more effort we make in searching, chasing and seducing a partner, the bigger the reward gets: the dopamine released by the reward-center of our brain goes higher and higher. No wonder both animals and humans go to extremes to win such reward, the euphoric drug that makes them go ecstatic. It's not surprising either that group sex or swapping partners is not a new fad; promiscuity between males and females had been the norm tens of thousands of years ago among early Homo sapiens.
Those who give themselves up to unrestrained sex, inevitably give up all reasoning too, just like drug addicts do. Only later they pay for the irrational acts triggered by their "addiction to sex": one of the most common and least discussed. The price can be an unstable life, unsuccessful marriages, and an impetuous risk-taking behavior generally affecting their health, finances, and relationship with other people. They, who simply followed their animal instinct and innocently pursued their wildest fantasies, only realized too late that the price of following nature was higher than the pleasure they received, let alone the damage it caused. Mother Nature wouldn't shed a tear for them, while watching them die one after the other, to accomplish her mission.
Not everyone chooses to follow nature on that genetically programmed route. Some choose a certain social system developed to organize the relationship between partners, for safer, deeper pleasure and less damage: they choose monogamy. Monogamy is NOT natural; yet it's the only choice for a sexually active person to enjoy mutual love, while saving themselves the bias, indignity, instability and risk, caused by the polygamous lifestyle.
There is no other way to achieve fairness in a long relationship than taming our animal nature to accept controlled monogamy as an alternative to the unethical, hazardous and insatiable polygamous lifestyle Nature wants us to have. If we choose short-term relationships instead, feeding our libido on short affairs, casual sex and one-night-stands, we objectify the other party, whom we manipulate to give us a physical thrill, till someone else "hotter" than them crosses our path and replaces them. Assuming that such thrill is safely achieved, still, its effect is temporary, with short-lived benefits and lasting damage (an analogy between casual affairs and fast foods perfectly fits here, in terms of pleasure, benefit and harm). Adopting this butterfly/grasshopper lifestyle eventually turns one into a chronic bed-hopper and their partners into mere sex-toys.
Otherwise, one may choose celibacy: having no relationships at all.
It's not wise or dignified to invest one's affection in a relationship where they are only needed temporarily, till they become replaced with another, for nothing wrong they did other than loving wholeheartedly.
Sex is very volatile and ephemeral by nature, and polygamy is too weak a foundation to build human relationships and society upon. Whereas in monogamy, where sex drive is both controlled and enjoyed, a person saves their energy for one partner to enjoy the benefits of "sex and friendship" together with, pleasing and bonding one partner to the other, to their children, and to the rest of society. In monogamy, your partner is "always" attractive, as long as you want, and "very" attractive as much as you want.
Not only multi-partnership is unethical, but the very nature of sex itself is too, thus needing constant vigilance, control and refinement of such basic animal instinct to be properly enjoyed. To avoid the harms of sex altogether, some individuals choose to do completely without it and become celibate!
If sex appeal is measured by "points," then some will be given to beauty (symmetry), gender features (level of masculinity or feminity), good personality .... but none of them has any use without the type, the main cause of attraction, that is very personal and subjective, varying from a person to the other: it's the way our brain is programmed, to like someone or not. Programming or de-programming the type may take months, years or decades, mostly happening unconsciously.
Turning a committed relationship into an open one opens the door for our primitive brain to try every new type, color, race ... until we become fully depleted. Sleeping with a different partner doesn't give us much experience or knowledge, as nature fools us into believing. Having a new affair with every poem you write, picture you paint, country you visit ... doesn't increase your knowledge. It only induces a temporary effect caused by our body chemistry, to act as if under drug influence. Not before long, you wake up to reality, finding your infatuation with the one you have been chasing is mysteriously gone, forever.
• Polygamists argue that their relationships transcend jealousy, selfishness and possessiveness that monogamy has. Thus, they consider themselves trans-humanists, and polyamory as a more "evolved" form of human sexuality.
It is true that some couples in monogamous relationships suffer some of the above, basically because of insecurity. When monogamy is not properly protected by laws and individuals, it becomes fragile and easily broken. Many couples forget, early in their romance, to consider protecting their relationship. They enjoy only the here-and-now of love's delights, which is all they see, without planning or preparing for any future changes. People do not understand the nature of monogamy, mistakenly taking it for granted and thinking it's natural, while it's not. To keep the pleasures of monogamy one has to make some effort first, to enjoy a secure, long-lasting relationship.
• Polygamists argue that polygamy has more emotional freedom than monogamy, thus suffering less emotional attachment, broken-heartedness, etc. In monogamy, you "put all your eggs in one basket"; thus, when you lose the one you loved, you suffer as much as you have loved.
However, you still suffer in polygamy from attachment too, especially close-ended polygamy; you only suffer differently. In polygamy, attachment is divided, which accordingly divides pain but does not escape it. Let alone that such attachment is unfairly, uncontrollably divided, constantly changing and shifting by whimsical urges.
Attachment is inescapable; you can't love someone without becoming attached to them, which is the case with all other human bonds. To free oneself from all forms of human bondage is to stop living with other people, else to consider them mere lifeless objects running into one's orbit.
• Another argument polygamists use is that if we are polygamous by nature, then we should follow our nature.
This is a weak argument, since following nature doesn't always make us happy. Actually all human civilizations are not natural; they are man-made. Man creates his own happiness; nature doesn't serve us pleasures on a silver platter everyday. Look at what nature does to all the people dying everyday by natural catastrophes, and how we all live at the mercy of nature's laws of physics. Not to mention that, if we follow nature restrooms will become obsolete!
• There is another argument for polygamy, one not based on sound logic but on a logical fallacy, by "appeal to novelty." Polygamy's novelty makes it a blow to many social conventions, particularly the millennia-old institution of marriage. This is fallacious because being new doesn't necessitate being right. However, being old, in the case of marriage, doesn't guarantee being right either. ONLY experience can decide which lifestyle to choose—but haven't we experimented enough? Polyamorists should go back to human and natural history books to see how the time humanity spent experimenting with polygamy is much longer than that spent on monogamous relationships.
Many zealous futurists like Robert Heinlein and Aldous Huxley like to think of a humanity's future where all taboos disappear, where incest, polyamory, pedophilia, etc. become just part of everyday life. It's understood that their dream world of sexual liberties is a natural reaction to the sexual suppression they endured during their time (which is sadly still present in many parts of the world, causing many people to go from extreme chastity to extreme lewdness).
The best method to predict humanity's future and the future of ethics is, as always, using scientific facts. Choosing a lifestyle based on suppression or fantasy only is misleading, as either leads to wrong choices.
• By the pledge two people make publicly or privately, they better control and "drive" their sex drive, the latter being a great source of pleasure, as well as misery when abused/overused, turning into a negative force in their life taking its abuser in any direction, engaging him/her in affairs whenever with whomever. By monogamy's agreement a partner's libido won't be squandered on casual temporary affections.
• Thus, with one partner there is less insecurity about one's health, finances, and social and emotional stability.
If one makes a list of all forms of relationships based on sex, where libido is the main character playing behind the scenes (subconsciously, Freudianly speaking), the list goes: monogamy, bigamy, polygamy, open-relationships, swinging, couples for couples, singles for couples, etc. Of all the above, monogamy goes further beyond the present moment, where more attention is given to achieve maximum pleasure and benefit obtained at present as well as in the future, when partners grow older.
• There are more pleasures to share with a long-term partner who knows better your likes and dislikes than one you meet casually, hence more harmony within the details of your physical and romantic relationship: more sympathy and understanding, and less awkwardness and accidents you are not prepared for.
• Every human is different: the longer you spend with someone the more things you discover about them, fining how unique they are compared to other members of their gender. You enjoy an ever-growing bond between the two of you, nourished by such "special things" he/she alone has that distinguish him/her from anyone else, which you unconsciously become more attached to and comfortable with.
In marriage, there isn't any deep relationship but one. Juggling several relationships simultaneously, one loses control over the depth of each, as each is different and conflicting with the others. It's not like friendship, where one can use reason in choosing and keeping one's friends. Reason and chemistry are both constituents of any relationship, where we unfortunately can control the former but not the latter. So, happy or sad, we have no choice but entering the cage of monogamy—a prison we love.
• Polygamy is more exciting than monogamy "at first glance." However, after a while, when the thrill is gone, only with someone available, trustful and caring, can it be re-ignited anew. This cannot be done except in monogamy. Monogamous couples can easily "fake" polygamy's initial thrill by a variety of tricks to enjoy alongside the longer pleasures. They become better, more skillful players of "love game."
• Marriage is an old institution achieving harmony between society's members in a hierarchal order that promiscuity cannot achieve. The fact that it has survived for millennia is not without a reason. It's not society, traditions, or superstitions that made it endure; it's simply humans' desire to be happy.