Human behavior varies a lot. As compared with other primates, we're heavily influenced by culture, religion, family upbringing, and so forth. As a consequence, it's logical to conclude that our fitful monogamy is purely culturally induced and not instinctual. (On the other hand, we readily seem to accept that promiscuous tendencies are wired into our brains.) In fact, we are programmed to pair bond—just as we're programmed to add notches to our belts.
By programmed, I mean that our brains are set up so that we engage in these behaviors with a lower threshold of enticement than we would otherwise. Both these programs serve our genes, as does the tension between them. For example, on average, we stay bonded long enough to fall in love with a kid, who then benefits from two caregivers. Then we may easily grow restless and seek out novel genes in the form of another partner. Italian research, for example, reveals that our racy "honeymoon neurochemistry" typically wears off within two years.
Pair bonding is not simply a learned behavior. If there weren't neural correlates behind this behavior, there would not be so much falling in love and pairing up across so many cultures. The pair-bonding urge is built-in and waiting to be activated, much like the program that bonds infants with caregivers. In fact, these two programs arise in overlapping parts of the brain and employ the same neurochemicals. The Coolidge Effect (that sneaky tendency to habituate to a familiar sex partner and yearn for a novel one) is also a program. The fact that our breeding and bonding programs often dominate one another doesn't alter the fact they both influence us.
Even when we override inclinations like these, they lurk. So it is that mates must often grit their teeth if they choose to remain faithful in the face of urges to pursue novel partners. Such instincts are powerful. For example, most humans are wired with powerful parent-child bonding impulses, even if they choose not to have children. It is a rare mother who does not bond with her kids (although it can happen if, for example, drug use has interfered with her neurochemistry). Similarly, people may choose never to engage in sex and orgasm, but groups of interconnected neurons are ready to give them a powerful experience if they do.
Again, such programs are present because of the physical structures in the brain—especially those that make up "the reward circuitry." This mechanism is activated by a neurochemical called dopamine (the "I gotta have it!" neurochemical). This is why falling in love, sex, nurturing a kid, and, often, pursuing a novel partner all register as rewarding.
Without this neurochemical reward, pair bonders wouldn't bother to pair bond. They'd settle into the usual, promiscuous mammalian program, in pursuit of its rewards. Predictably, there is evidence of unique brain activation in pair-bonding voles (compared with the non-pair-bonding variety). And there is data showing similar brain activity in pair-bonding primates. (See: Neural correlates of pairbonding in a monogamous primate). Although more research is needed, it may be that pair-bonding mammals (unlike non-pair-bonding bonobos, for example) share similar 
neural correlates: neural networks, receptor type and specific neurotransmitters, etc. Neuroendocrinologist Sue Carter expressed this view: "The biochemistry [of bonding] is probably going to be similar in humans and in animals because it's quite a basic function."
While all mammals find sex rewarding, pair bonders also register the individual mate as rewarding. Thanks to this hidden pair-bonding program, our brains light up so we become infatuated. And our hearts ache when are parted from our sweetheart. Pair-bonding voles, too, show signs of pining when separated from a mate.
Need more evidence? Consider the hellish fury that arises when we are jilted for someone new. A cow, on the other hand, is quite indifferent if the bull that fertilized her yesterday does his duty with her neighbor today. Lacking the requisite neural correlates, she is not a pair bonder.
Given the fact that the urge to switch partners so often overrides our pair-bonding inclinations, shouldn't we continue to give this unreliable bonding program scant attention? Maybe not. Even though our pair-bonding urge is clearly not a guarantee of living happily ever after with a lover, a better understanding of it may furnish important clues for relationship contentment, and even greater well-being.
Orgasm's Hidden Cycle
It may be that if we want to sustain our unions harmoniously we need to understand more about how our sneaky genes create disharmony between lovers—in order to cruise into the future aboard as many vehicles as possible. This means learning about the neurochemical shifts that govern breeding and bonding, which occur primarily in the limbic system of the brain. They are similar in most mammals, and not under conscious control. They have been conserved by evolution because they lead to greater genetic variety and lots of progeny.
Genes never sleep. Instead of a blissful "they got married and lived happily ever after," gene fairy tales end with offspring and more offspring—any way the genes can get them. As surely as they drive couples down the wedding aisle in the first place, our genes will push us toward betrayal whenever infidelity is in their interest. Burnham and Phelan, authors of Mean Genes: From Sex to Money to Food, Taming Our Primal Instincts
Genes never sleep. Instead of a blissful "they got married and lived happily ever after," gene fairy tales end with offspring and more offspring—any way the genes can get them. As surely as they drive couples down the wedding aisle in the first place, our genes will push us toward betrayal whenever infidelity is in their interest. Burnham and Phelan, authors of Passion, it turns out, is a two-edged sword. It drives us together, and it can drive us apart. It releases such an intense blast of neurochemicals that one scientist compared the brain scans of men ejaculating to the brain scans of people shooting heroin. This intense stimulation has the power to desensitize the brain temporarily. That is, it dampens our pleasure response for a time, probably through mechanisms such as a decline in dopamine receptors in a primitive part of the brain known as the striatum (a part of the reward circuitry). Deep in the brain, it's as if the scales are tipped until the brain recovers.
There's much still to learn, but it looks like a number of reward circuitry events occur after climax that have the potential to desensitize us for a time. First, androgen receptors decline after ejaculation, and take up to seven days to normalize. (That means the effects of testosterone on the reward circuitry are probably blunted for a while, quite possibly affecting outlook.) In addition, opioids released during copulation hang around for a while, apparently causing lingering declines in oxytocin, which hamper sexual responsiveness. As noted above, there is also likely a drop in responsiveness to a neurochemical vital to our sense of well-being: dopamine. In effect, the brain has changed. It now requires more stimulation to get the same pleasure response as before, and sometimes no amount of stimulation will truly satisfy—until it recovers. Anecdotal evidence suggests that such changes affect both sexes.
UPDATE (2014): Researchers have shown that ejaculation, even twice a week, shrinks the dopamine-producing neurons in the VTA, which reduces a male's response to the pleasure of morphine. This also happens with repeated heroin use in both rats and humans. Scientists have also learned that amphetamine activates the exact same nerve cells as orgasm, urging the brain to "remember and repeat." In other words, drugs of abuse "work" because they're hijacking the mechanisms that evolved to make us want sex...and yet experience aftereffects from too frequent sex. For more research see What Do the Scientists Have to Say?
Whatever the precise mechanisms, a decrease in the pleasure response of the brain is big news for lovers. For one thing, not everyone experiences the recovery from an exciting wallop of neurochemicals precisely the same way—thanks to genetic and gender differences, childhood trauma, or their own habits.
Some of us are simply uninterested in sex until our brains return to their natural sensitivity, and orgasm once again seems like a great idea. In others of us, however, the temporary neurochemical (or receptor) drop-off soon makes us feel like we are missing some essential ingredient for our happiness. We are: our ideal sensitivity to pleasure.
The resulting angst strongly motivates us to seek relief now (i.e., go after more). Due to this "mini withdrawal," we may feel anxious and emotionally distant—and want to ease our tension with another orgasm as soon as possible. Or perhaps we are needier than usual, craving additional proofs of our mate's love—on our terms. Both these strategies are attempts to stimulate feelings of pleasure in a now-sluggish reward circuitry.
Unfortunately, unless both of you happen to choose the same "meds" for your discomfort, your love life can go out of sync. If your mate rebuffs your advances, it may seem like your mate doesn't care enough to please you. Or it may seem to your mate like all you care about is "getting some." Now, you could be seeing the worst in each other, and, perhaps, doubting each other's devotion—all because your mindless, primitive reward circuitry is giving you imperfectly matched impulses as your brains return to equilibrium.
Some of us naturally attempt to equalize out-of-sync libidos by turning to potent sex-aids and masturbation. Unfortunately, today's vibrators and extreme porn videos do their jobs with such intensity that they further over-stimulate and desensitize the brain. After a time, the thought of normal sex with a familiar partner may no longer power up the urge to merge.
What to do?
Obviously, the curse of sexual stimulation leading to desensitization and discontent is not a new challenge. Two thousand years ago, Roman poet Ovid advised the following cure for love: "Enjoy your girl with complete abandon, night and day—and loathing will end your malady."
This may be why sages across the globe developed techniques for managing sex
, which keep lovers in balance and sustain the harmony of their unions. Kosher sex, for example, prescribes almost two weeks a month in separate beds. Methods such as karezza and Daoist dual cultivation call for frequent lovemaking without the emphasis on orgasm. Such strategies help couples retain their sensitivity to the nuances and bonding power of warm affection, leading to deeper contentment—and less sexual frustration.