Consummation, as set down dispassionately by the Oxford English Dictionary is "the action of making a marriage or relationship complete by having sexual intercourse"; from the Latin verb consummare, uniting con- (altogether), with summa (sum total), the feminine of summus (highest, or supreme). An end, then, that may be no less a beginning and a lofty one at that, ripe with connotations that the act itself should prove consummate.

And yet, in practice, sex is rather akin to dancing: something physical and exuberant that makes most of us look like arses. Tinseltown intercourse certainly packs a terrific bang for its buck: so many tastefully lit montages of deeply meaningful acrobatics. (What was sex like before celluloid, the post-modern citizen wonders, before orgasms become as platitudinous as Meg Ryan herself?)

Back in the real world, we may dress for sex, hanker after it, thrill and scheme, but the reality is that the fist slamming, raw-faced, ululating meeting-of-minds-as-bodies phase can be frustratingly short-lived. As the American writer and wit Anita Loos decreed: "Sex, which has been acclaimed by too many misguided poets as an utopian activity, seldom attains that status in the human race." Like life, it has a tendency to be nasty, brutish and short, which may or may not be exactly how one likes it.

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Too much that falls under the category of consummation conforms to the nursery rhyme dictum that when it is good it is very, very good, but when it is bad it is horrid. There are too many permutations. The anonymous encounter whose ghost looms like a priapic Colossus over congress with someone you actually like. The lacerating, genitals-only connection with someone you once loved. The absurd, two-person pantomime that can have us writhing with mortification years in its wake. As a friend once implored: "Why only beer goggles, why not lifelong blinkers?"

One of the most popular novels of recent years, Ian McEwan's On Chesil Beach, might well be a record of non-consummation, of a honeymoon encounter turned into the dampest of squibs. The British have yet to shake off entirely the impression that sex falls somewhere between the territories of Philip Larkin and Benny Hill: something that is both ignominious and comical.

A run-in with the website beautifulagony.com provides ample fodder for those of this persuasion. The site is a picture library of the facettes de la petite mort (or "cum faces" in the lexicon of the lavatory wall) of its "agonées". "The only nudity it contains is from the neck up," declare its creators. "That's where people are truly naked." And so – gurning or beatific– they are. What is at once most brilliant and terrible about sex is its capacity for self-exposure.

Somehow intercourse makes us feel as if we are revealing something at the very core of our beings; even if at this core lies merely more play-acting. Sex is the most resplendently subjective experience, which implies there is a subject, or so the logic goes. The rhetoric of congress reflects this assumption: an individual becomes known via the sexual act.

And there is a sense in which we all recognise the veracity of this: the rush of something passing for post-coital intimacy; that intoxicating alliance born of pillow talk; the sudden, potent familiarity with someone who was until recently exhilaratingly unknown; something unfamiliar discovered in a familiar ally; the mess of selves and sweat. You and me against the world, if only for the time it takes us to gather our wits.

And, yet, of course, these sexual selves are as constructed as anything else our consciousnesses can muster. Unravel the tangle of many a woman's thwarted idea of arousing feminine behaviour and you will discover some chimera made from: Scarlet O'Hara, Elizabeth Bennett, Holly Golightly and "Not My Mother"; ready to drop her knickers for that elusive Rhett /Darcy /Peppard / daddy substitute.

Someone's particular fetishes may be less knowingly laid down: the thumbsucker who yields to more graphic oral fixations; the squirming schoolboy who grows up ever hankering after a walloping; the News of the World journalist who goes hard at the prospect of a Mosley in uniform. Some epiphanic moment spawns a rubberite here, a frottage enthusiast there, our sexual selves becoming all this and more, greater than the sum of their, or anybody else's, parts.

Animals that we are, we sniff our lovers out. A significant factor in whom we find alluring is that we succumb to the scent of genetic compatibility or, rather, the complete lack of compatibility that ensures a healthy genetic mix. But this is only the beginning of our substance abuse, because sex is chemically addictive, rendering us all crack whores.

The initial flood of lust feeds off the gendered part of the package, notably a torrent of testosterone and oestrogen. This is the pre-consummation stage, or the period when consummation has been so fleeting as to leave no lasting mark. The high it inspires is a gas, but not yet a dragging compulsion. The ensuing love or infatuation phase relies on a potent cocktail of neurotransmitters including adrenaline, noradrenaline, dopamine, serotonin, and phenylethylamine – or in layman's terms, an entirely head-fucking bolt of stimulants.

This is a havoc wreaked in consummation's wake. Where lust was a diversion that could be kept in its box, so the junk of infatuation consumes us. Renaissance poets recognised its ravenousness in their obsession with the erotic contortions of Ovid's Metamorphoses. Robert Burton's early 17th-century treatise, The Anatomy of Melancholy, devotes a considerable portion of its musings to libidinous frenzy. A century on, the vernacular take on such dependency was more spade-calling, the afflicted deemed "cock-struck" or "cunt-struck".

Most of us will have been so stricken. Something bestial crawls under the skin, rendering us moony, jittery, distraught and rhapsodic. Overnight we have the energy for a range of behaviours we would hitherto have strained to accommodate: Herculean erotic feats, the trading of interminable biographical detail, cavernous sleep deprivation, texter's finger, shagger's groin. We become nearly as compelling to have about as the coke addicts from whom we are scarcely distinguishable. The love object's every gesture is analysed, every adjective applied to them qualified, every opportunity to refer to them clawed.

Tedious, yes, but what is most crushing about this chemical high is how fleetingly the fix endures. The affection or commitment stage that follows this honeymoon period, typically at around 18 months, is the product of the hormones oxytocin and vasopressin. Both are believed to interfere with dopamine and noradrenaline pathways, heightening bonding, but sabotaging sexual craving. Hence that most painful of erotic paradoxes: at the point where you have never have loved someone more, you may never have wanted to sleep with anyone less. Familiarity may not breed contempt, but it is likely to guarantee sexual torpor, as a million magazine articles testify.

Chemically, then, sex may be mere sex, or it can seem like everything – after which it seems destined to become a chore. Small wonder that we should chose to invest consummation with so much meaning, ham it up in inverse proportion to the brevity and insignificance of the act itself.

Consummation's past associations have been legion: disease, danger, fertility rite, sin, the accruing of dynastic swagger. It was not until relatively recently that sex has been thought to coalesce with wedlock, wedlock with love, and love to loop back again and lasso itself to sex. Frances Osborne's engrossing biography, The Bolter, offers a caution against the impression that our grandparents' sexual mores were more rigid than our own. Meanwhile, The First and Second World Wars unleashed orgiastic eruptions of extra-marital copulation.

Nevertheless, by and large, as the 20th century wore on, consummation came to signify some sort of mystical, marital union of simultaneously orgasmic soulmates (thank you D H Lawrence and Marie Stopes); a fairy-tale that endured long past the sexual revolution that was supposed to have set morals swinging.

If Freud made sex a panacea, Lawrence lent that panacea transcendence – a surrendering to the body to escape its bounds. Bona fide Lawrentian union – proud, elemental, phallic, unselfconscious – was to be favoured over "sex in the head" – visual, voyeuristic, controlling – beloved of "cocksure" females and other deviants. "Why were we driven out of paradise?" he inquires in Fantasia of the Unconscious. "Not because we sinned. Ah, no. All the animals in Paradise enjoyed the sensual passion of coition. Not because we sinned, but because we got sex into our head."

A century on, and postlapsarian sex in the head is the only show in town. While various orifices may be involved, our over-stimulated brains are where it's really going down. Consummation is not epochal, but recreational, as entertaining and banal as any other leisure activity. Sex can be pursued as a team sport, a game of doubles, or a solo exercise. Inanimate objects have become fair game as – in a reversal of pre-20th century anxieties – self-pleasuring has come to be regarded as a purveyor of mental and physical health ("I use my vibrator to give my skin a boost," remarks an acquaintance with the nonchalance of the salubriously orgasmic heroine of Brave New World).

Our culture is not so much navel-gazing as fixated further south. The over-fifties gad about amassing STDs, their offspring hooking up – a fuck not so much zipless as conducted at emotional arm's length, less coitus than mens interruptus. Sex is considered safe if it precludes propagation or disease; an underestimate of the damage one can do while one's wedding tackle remains intact.

The true fetishist has become he or she who abstains from consummation while the rest of society gets its kicks in a post-modern pornutopia in which all manner of experiences can be had with all comers and myriad things. The celibate are stigmatised as freakish, thwarted. And yet the great surveyed platitude among consenting adults is that most would rather sleep than screw; a kink at which uncontracepted generations would surely have gawked.

Perhaps the same chemical flattening that dulls intercourse in long-term unions applies to society at large. Where there is opportunity, satiation, multiple consummations, so there must also be over-familiarity, ennui. The terrier-like, up-against-the-leg sexual enthusiasm of mainstream culture also brings collective apathy in which all rush is spent.

And yet, in spite of all this, sex still has the ability to knock us sideways: that element of voodoo rising that can fell at a hundred paces and have even the most po-faced mouthing Molly Bloom's ecstatic: "Yes, yes, yes!" Its sway is registered on an epic scale from the fall of Troy to the detumescence of the Clinton White House. It imprints itself in our lives like a post-coital bruise. For, suddenly, here you are, stumbling across someone with whom you are not an arse, but dancing with the agility of Sylvie Guillem, to collapse in a great wash of elation, exhaustion, gratitude, spent rubber and, still, every now and then, love.