There was a distinct moment when it all changed.
We spent our teenage years discussing boyfriends in every gory detail. As we grew older, we whispered about emerging tastes and compared notes on techniques. But then, in our twenties, we all began to settle down. And suddenly, our sex lives became our own business, thank you very much. We didn't want to talk about it any more.
Certainly, we'll gossip about affairs and roll our eyes at the hard labour of trying to conceive. But we draw a discreet veil over the actual sex. By and large, women are expected to imply they're completely disinterested, and men are obliged to look hang-dog and deprived.
It seems to me that this hides a hugely complex picture. Sex is often a fraught issue for couples, with problems such as health, ongoing disputes, workload, children and boredom affecting frequency, if not quality, in the bedroom. Meanwhile, our equal modern relationships leave many of us struggling to make the transition from best friend to lover when the time is right.
Perhaps this is why we become so secretive. We enter into these long-term relationships with an armoury of sexual experience, and a few years in, we're bewildered and hurt at the sense that sex has all but disappeared from our lives. We're too ashamed to talk about it, assuming that everyone else is rubbing along just fine.
It was certainly true to me. After 15 happy years with my husband Herbert, my marriage was an erotic graveyard. Attempts at confiding in friends had hit the buffers.
When we decided to turn it around, by taking it in turns to seduce each other once a week for a year, I was certain there would be other people out there just like us. And as I started to blog about our project, I found my instinct was correct.
As we limped our way through the first few months, men and women alike contacted me through my blog or on Twitter to share the triumphs and tragedies of their own sex lives, or sometimes just to ask, 'Am I normal?'
I came to rely on this two-way flow of information to get me through the seductions. In exchange for my willingness to delve into the nitty gritty of married sex, people I had never met before offered tips, played cheerleader when I was flagging or told me off when I was being ridiculous.
After a somewhat soggy attempt at role play, for example (Herbert had dreamed up two of the least sexy characters imaginable, a tie-dye wearing older woman and a young virgin so shy that he could barely spit his words out), I received a flood of messages, some offering better ideas, some consoling, but not a single one mocking me.
It felt like a luxury to be able to talk about our experiences in detail, without pretending that everything was perfect or feigning embarrassment. The pact of secrecy had been broken, and we were all grateful for it. Married people aren't dried-up rusks, but adults with fears, desires and tricky situations to confront.
For me, the biggest secret of all was that I felt incompetent around sex, even after all these years. I felt as though I should have acquired some expertise, and I was too shy to admit I was a learner. Over the course of our year of seductions, I slowly dismantled this deadening assumption.
And in the process, I have become quite evangelical about the need to discuss sex in long-term relationships, whatever shape it's in. After all, it's the worst-kept dirty secret out there.