"Heterosexual couples get together, often when young, drawn by this strong mutual erotic and romantic attraction," she says. "But over time - after children, the menopause, daily familiarity - the sexual element fizzles out."
It's a scenario many may find themselves quietly recognising. But it's not surprising, says Roffey.
"In the West, we have developed this idea over the past century that marriage has to be all-encompassing. It has to be sexually profound, but your partner must also be your best friend, and this must last for these long lives. It's an incredibly hard thing to achieve.
"How did this impossible promise at the altar get written into societal consciousness?"
Marital celibacy and sexual freedom are subjects 52-year-old Roffey, a lecturer at Manchester Metropolitan University, explores in her latest book, The Tryst, an erotic novella about a sexless - but outwardly perfect - marriage. The union is shaken to its core when wife Jane and husband Bill decide to "spice up" their relationship by inviting a strange woman they have met in the pub to their home and bed for the night.
She explains: "Bill and Jane have both been burnt by erotic affairs in the past and now feel they have found sanctuary in their deeply loving, albeit physically unfulfilling partnership. He has married his mother - a strong, self-contained woman. Jane has chosen in him a father figure. No wonder they haven't reached their erotic potential as a pair."
Instead, the couple become easy prey for a magical sexual predator who has no intention of being just a one-night diversion to their marital life.
Although the book is not autobiographical, the loss of passion within a relationship is one Roffey understands well; a long-term relationship with a fellow writer ended due to his infidelity in 2006, when she was 41. "We were a great couple," she says, "in a loving-but-sexless long-term relationship.
"Looking back, I was so young then. I did not know I had got myself into the trap, and didn't know what to say or how to handle it, or even how to get out of it."
Which is not to say she was or is some sort of shrinking violet. Born in Trinidad to European parents, Roffey - a rather majestic figure, with a mellifluous deep voice, sharp brown eyes and a glorious abundance of hair - was sent to a convent boarding school near Weybridge, south-east of London.
"I was one of the naughty girls, really rebellious. I'd shimmy down the drainpipe to get out; it was St Trinian's really."
Holidays were spent back home in the Caribbean hanging out with her brother and their local friends. After studying English and film studies at the University of East Anglia, Roffey went into journalism, before falling ill with a rare auto-immune disease called Churg-Strauss Syndrome at the age of 30.
"I was in hospital for months. It took me a year and a half to get over, stopping me in my tracks."
It was possibly the life reboot she needed, and Roffey was inspired to begin a creative writing course in London, and then take an MA at the University of Lancaster in 1999, where she met her ex-partner. The pair went on to run an Arvon Foundation centre for writers in Devon and worked on their own books: "I used to see us as a literary union, like Sylvia Plath and Ted Hughes. I'd sit and watch him work. He had such grit."
The relationship ended when Roffey received a package in the post containing dozens of printed emails, as proof that he had cheated on her. Life as she knew it was, understandably, "blown apart".
She fled home to her mother in Trinidad, and wrote her way out of despair. The resulting novel, The White Woman on the Green Bicycle, was shortlisted for the Orange Prize and the Encore Award. Her next book, Archipelago, won the OCM Bocas Prize for Caribbean Literature.
But when the writer returned to London later that year, she was still suffering "post-break-up shock and grief". However, rather than shut herself away, or look for another serious mate, Roffey decided to explore her own erotic needs. "I wanted to have sexual adventures. I advertised on Craigslist for no-strings-attached sex. I was crazy, wild. My friends were horrified, titillated and shocked.
"For someone from this vanilla, heteronormative background, it was eye-opening. I was on my own journey of sexual education. And gradually I learned to combine intimacy with exploration.
"I feel now I spent the past 10 years getting out of the mainstream into this marginal sex positive/queer space I now inhabit." By which she means she is single, not celibate, and her sexual interests are "broad".
She adds: "At 52, I am very comfortable with where I am. I had to hack through the jungle to get here. But I know that mainstream society might look at me, and wonder why I'm not married, because I don't have the status that we are programmed to desire and capture."
This question of whether many women will continue to suppress erotic desires in order to remain in otherwise-happy marriages is one she comes back to. "At 40, I was deeply dissatisfied with where I had got to and what had come my way. As a young woman, I hadn't had what I wanted. I wasn't sexually alive."
But how many women could, or would do what she did? "I'm a bohemian, and one of life's natural experimenters," she acknowledges. "Until now it was hard for women to explore themselves, it was taboo. But things have changed in recent years."
Roffey's new life offers freedom but challenges, too. "Like all child-free women, I have to decide how to spend the next half of my life. To think, what now? I don't have an answer. When women get into their 50s and 60s, there is this sense you are moving off the script. Keeping writing is a given, but I've also started life drawing at art school."
She adds: "I look around me. That script, I tried to live it until I was 40. I had a 'marriage-type relationship', a home, a conventional job - I tried to fit into the mould.
"But it broke, and I went somewhere different, I didn't come back. Even now, I've been single for two or three years but I'm hoping there's more out there. What I do know is that whatever my next relationship looks like, it won't be that heterosexual norm. There are still more rocks to turn over."
So does she have any words of wisdom for the Bills and Janes caught up in loving celibacy?
"To any couple out there who have fallen into a sexless relationship, I'd say the first thing to do is find out the problem - if it indeed is a problem - and talk with compassion to each other. Female sexuality is complex. Talk, seek help, or you could try a Tantra workshop together."
Perhaps even venture on to the internet: "It's not all seedy porn and low-life. My advice is to log on and name your desires and see what comes up. Get with the programme. It's all there. Seek and you shall find. And take your partner with you."
The Tryst by Monique Roffey (Dodo Ink, $16).
