The loss of sexual ignition is a simple, unavoidable and bewildering fact.
There is a universal truth to all long-term sexual relationships that is never openly discussed. This widespread sexual malaise lurks behind all our frenetic sexual questing and provides a feast for all the opportunists, from philanderers to pornographers to Bettina Arndt.
It is simply this. At some point every sexual relationship loses ignition. This simple, unavoidable and bewildering fact is a hidden infirmity that in our solution-compelled world we imagine we can resolve without first fully understanding.
We can't. Once sexual ignition dissolves - for some after three years, for others after 10 years, for a rare handful, years later - it thereafter makes rare appearances between the sheets. Many turn to what are quaintly termed ''marital aids'', ranging from lingerie and fantasy to toys, DVDs or acts that give that kick purely because they are transgressive.
Advertisement: Story continues below What do I mean by ignition? Ignition is the roar of the engine, the overtake of all senses and thought processes by the overwhelming need to get into your lover. Combustion might flash when the inside of a knee shows as it is crossed under a bar. It might spark within the first frank gaze of longing, the flick of a tongue, the wrench at a belt buckle. And when it ignites, every last one of us is clawing, thrashing, convulsing and doing flamenco hands with our feet. When it mutually ignites, sex becomes a head-on collision. It is, we've all said it, The Best Thing Ever.
Ignition is what we all want. But most of us have loved someone enough to stay all the way to the point when it is lost. Most of us have grieved it, some quietly, at the risk of making our beloved feel we have lost ''attraction'', even though objectively we know that doesn't make sense.
Some of us sensibly talk it through, knowing in doing so we destroy that entirely unbidden mechanistic jolt that by its nature can never be contrived consciously. Some of us endure rounds of counselling thinking ''intimacy'' is lost, even though objectively we know that also doesn't add up.
Ignition is something outside of words or cultural contrivances. I am a social constructivist, yet I think ignition is something we will never explain or manufacture. Ignition is essential, yet, like all the biological things we do with our bodies, we have built elaborate social rituals around it, from monogamy to conjugal visits to Viagra.
We can hypothesise into the next millennium about why ignition is lost - children, overwork, domestic inequity, impotence - but for now we haven't yet faced up to the fact that we all try to survive in sexually exclusive relationships without it. This is a very tough call. Margaret Mead wasn't the last to observe that the institution of monogamous marriage depends on wide-scale prostitution.
Couples who survive the loss of ignition have invested so much of their lives in their sexual inter-dependency - children, mortgages, love - they hobble along without it, since there is too much at stake to overthrow on such a seemingly facile premise. Make-do sex rules. Some replace it with other things and, though technically sated, they live with a certain hollowness.
Speaking, as I have all the way through, from experience - extrapolating outrageously in fact - those of us who find a way through the loss of ignition settle for sex without it because losing all the things invested in a life partnership is a far worse prospect. Besides, enduring love may not turn the key, but it can be immensely, ecstatically sating, and the glow goes on for days. This kind of sex oils the machine. It's great, and we accept that it doesn't roar.
But the loss of ignition is part of life so long as we construe our lives around monogamy. Monogamy can't sustain ignition. Generations ago I suspect ignition was less central to ideas of sexual success and successful relationships. Its loss may have been experienced as part of the maturing of a relationship from which, once endured, new things could be built.
Is it a matter of just growing up and getting over it? I for one think we humans weren't meant to be sexually exclusive, and building our lives around monogamy has been disastrous for too many, particularly when children are involved. How long before we acknowledge it's not working, it rarely has and it rarely will.
Perhaps the problem isn't the loss, which we should openly accept as part of the pact of monogamy - though preferably not on Bettina Arndt's website as something mean-spirited women inflict on men. The problem is we repress the loss of ignition and don't know how to tolerate it and move on to something else. That something might be polyamory, or it might be some cherished understanding that binds a couple closer. I think after decades of sexual questing we can safely say it isn't anything the sex industry has been able to resolve for us.
Dr Liz Conor is an academic at the National Centre for Australian Studies, Monash University.