Of course, Gilbert is not the first female celebrity to leave her man for a woman. In 2003, Sex And The City's Cynthia Nixon shocked fans when she left her boyfriend of 15 years and father of her two children to move in with a woman.
Mary 'Queen Of Shops' Portas was married to a man with whom she had two children before meeting her fashion editor wife. And psychotherapist and writer Susie Orbach spent more than 30 years with the father of her two children before her relationship with writer Jeanette Winterson.
But I know all too well that this sort of about-turn is not confined to celebrities. Five years ago, I had a brief affair with a female friend and, though my marriage survived - thanks to a very understanding husband - I did face his initial confusion.
So why do so many otherwise heterosexual middle-aged women develop a sudden attraction to females? As I tried to explain to my husband - and my friend Giles - in many cases, the affairs have little to do with sex and everything to do with your time of life.
In my case, I was a 40-year-old mother of three who had never had so much as a teenage crush on a female before, never mind a physical relationship with one. I wasn't even attracted to women, and I still loved my husband, Erik, then also 40.
Yet I'd found myself caught up in a passionate affair that could have ended my marriage.
While I didn't leave my husband, I'm convinced a lot of women make the mistake of doing so because, in our 40s, our needs from relationships change.
We don't need men biologically any more as fathers, we've often raised our children and we find we have more in common with other women than with men.
Sometimes, this spills over into a physical relationship and women who have never considered themselves lesbian find they 'fall in love' with their female friend. But I think it's a mistake to make the relationship sexual as it changes the dynamic and, often, we lose the friendship when the romance dies. As happened in my case.
Women need to recognise their emotional needs have changed and find ways to share female friendships without making the error of thinking it's sexual love.
I had become too settled in my cosy, middle-class life and lost the adventurous edge I'd had when Erik and I first met.
My affair taught me some surprising things about myself, and jolted me out of my rut. But I was lucky: my husband forgave me. It's not a risk I'd take again.
Erik and I met in 2001, when I had flown to Florida for a friend's wedding. Bruised by an acrimonious divorce, romance was the last thing on my mind.
But I got talking to a tall, handsome American with blond hair and blue eyes and fell for him straight away. He was studying for a philosophy PhD, while I was a journalist in London.
In my case, I was a 40-year-old mother of three who had never had so much as a teenage crush on a female before, never mind a physical relationship with one. I wasn't even attracted to women, and I still loved my husband, Erik, then also 40. Yet I'd found myself caught up in a passionate affair that could have ended my marriage.
We flew back and forth across the Atlantic for nearly a year, and then, in 2002, I moved with my sons, Henry and Matt, then seven and four, to Florida permanently.
Our daughter, Lily, was born the same year, and we married in a church in December 2005.
Three years later, we moved to Vermont for Erik's work. I loved our new town, and the children settled well into their schools, but once more, I had to start again finding new friends and I was often lonely.
I worked from home, which made it hard to meet new people. I was used to living in a city, and felt suffocated by the small town where everyone knew each other.
Much as I loved my husband, I craved the kind of intense feminine friendship most women take for granted. Then, out of the blue, I got an email from a former colleague who had moved to Montreal, around two hours away in Canada. I didn't know Freya well, but she was also British and far from home - so when she suggested meeting, I agreed.
We clicked straight away. It was so nice to have a girlfriend to talk to. Erik was always sympathetic, but there are some things only a woman can understand.
We met every few weeks, and then she invited me to a party in Montreal. It was wilder than I was used to, and we drank too much. At some point, Freya kissed me and we ended up in bed.
Inhibitions long gone, part of me was simply curious: I'd never done this sort of thing before. But it also felt surprisingly natural.
And because it was a woman I slept with, not a man, I tried to kid myself I wasn't unfaithful. Sex with Erik was still satisfying, but over the years, my interest had dwindled and we were in a rut.
Freya and I grew closer over the next few months, talking nearly every day on the phone and texting. We were both entering our 40s, dealing with teenagers, coping as freelancers in a depressed economy. With her, I didn't have to explain how I felt.
I realised I was confiding in her instead of Erik. I felt too guilty to tell anyone about her, and I was acting like I was having an affair.
Psychologist Lisa M. Diamond, of the University of Utah, and author of Sexual Fluidity, argues that, for some women, love and desire are not rigidly heterosexual or homosexual and change as women move through life.
"One of the fundamental, defining features of female sexual orientation is its fluidity," she says. She believes women are more open in how they connect to others and that, sometimes, when a woman forms a strong emotional bond with another woman, sexual attraction can follow.
I know two other women who have had similar experiences, but their marriages didn't survive.
One is now happily living with her lesbian partner, but the other bitterly regrets blurring the boundaries of friendship and sex.
In my case, when I told my husband, he was remarkably forgiving, revealing that he'd known about the affair all along, but had been waiting for it to run its course.
I'd thought he wouldn't understand what it felt like to be a woman entering middle-age, prey to hormones and the ravages of time. But, as Erik put it, he lived with me so he knew me better than anyone - male or female.
My assumption that as a man he wouldn't 'understand' me was as sexist as it was wrong.
Freya's husband, however, was livid and we stopped meeting.
Of course, there's nothing wrong with intense female friendships, but it's a mistake to confuse that with sexual love.
And, ultimately, an affair with another woman is still an affair. Betrayal, heartbreak, misery and disappointment will follow - and it might turn out to be a costly mistake indeed.
