'I'm thinking constantly about my gender." That's what my husband said one June night, seconds after making love. As post-coital murmurs go, this one was a knockout. Sex, among other things, would never be the same.

"I can't stop thinking about it," he said. "I keep feeling like I'm the wrong gender, a lot, all the time, constantly."

I don't remember how I responded. I know that I stayed surprisingly calm, for me. I heard the urgency in his voice and tried to be supportive, as I would often fail to be in the many conversations that would follow.

"I just want to talk," he assured me. "I'm not going to do anything." By which he meant, it went without saying, anything to his appearance. I was stunned. Our marriage, our family and everything that up until that moment had constituted our story was over. That much I understood at once.

Tom and I met and fell in love at college. After graduation we had various jobs. I wrote, we travelled. We got married, had a child, then a second and a third. The Tom I knew was sharp, funny and irreverent. He didn't come across as feminine. His signals were heterosexual and male. He initiated our intimate relationship and responded to me in the ways I expected. I can still see his look of stark sexual appreciation when he spotted me walking towards him on a date. When he told me once, early on in our relationship, that he hated himself and had sometimes wished he was a girl, I assumed it was psychological – a rejection of self. Tom had a difficult upbringing, so for me it was a given that what he meant was that at his lowest moments he had wished to be something he knew he was not.

Tom told me a few years later, early in our marriage, that he was struggling with these feelings again. I still thought he was investing gender with a power to resolve his childhood problems. But this time it hit me that he had at least contemplated cross-dressing. This understanding was so disturbing, it literally made me nauseous and dizzy. For me, there was no wiggle room: I couldn't engage in an intimate relationship with a man who dressed in women's clothes. Not even occasionally. Not even in secret.

Given the strength of my reaction, it may sound strange that I thought I could continue in the relationship – in hindsight, it does to me – but Tom had put aside these feelings. That's what I believed. I didn't think he had suppressed them; I thought he had let them go.

Over the years that followed, there were moments when Tom seemed distant and preoccupied, but for the most part we were in harmony. We took long walks, frequented cafes and bookshops, spent hours at home reading aloud, cooking and drinking wine. Tom was an avid football fan and he taught me the game so that I could enjoy it with him. He lived with my preference for what he called relationship films and I lived with his films involving aliens and violent death. We talked about almost everything. We had every conversation, except the ones we didn't have. We never spoke of the discomfort Tom had once expressed about his gender – but those feelings had been resolved long ago, hadn't they? And we didn't talk about sex. For more than two decades, we had an active and, I believed at the time, satisfying sex life. We didn't sleep in separate beds. We didn't forget to touch, didn't find sexless weeks slipping by unnoticed. But we never said much about it.

'Oh, look," I said to Tom one day. "This is perfect for you." I was reading the events calendar of the local newspaper. "A Jewish men's group. Why don't you check it out?"

"I have no interest in that," Tom snapped. "What would I have in common with them?"

I laughed. He was kidding, right? What did he have in common with his own demographic? Tom wasn't kidding. He was angry, as insulted as if I'd suggested he join a group for the mentally impaired. "I don't want you making suggestions like that," he said testily.

Yet Tom was interested in my relationships with other women. Too interested. Whenever I began a friendship, he would edge suffocatingly close. One time he called a new friend in secret to ask for babysitter recommendations so he could take me out for my birthday. After that, he often found some pretext – it always felt like a pretext – of doing something nice, and got his hands on a friend's phone number, calling for advice or information and asking her for secrecy. It felt creepy every time.

At the time, though some friends didn't know what to make of him, most thought Tom sweet, gentle, the sensitive type – qualities that, when I encounter them in my friends' husbands, now cause me, entirely unfairly, to cringe on my friends' behalf. As if I could see the knickers, the tweezers, the boat-sized high heels heading their way.

That night, after Tom's announcement, I tried to believe that our life together was going to continue, because, quite simply, I couldn't believe that it would not. Tom had a psychological problem, a big one. We would find a way out of it. What other choice was there?

The next afternoon we took a walk on a winding country road, with Lilly, not yet two, in the buggy, and Adam and Bibi on bicycles. When the older kids were out of earshot, Tom repeated the salient points of the previous night's conversation. He felt wrong in his body. Increasingly so. It had gone from being an occasional thought to a constant state of mind. An obsession. "I can't stop thinking about it," he told me. "Sometimes I feel like I'm not going to be able to function."

Tom mentioned that he wanted to find a therapist, possibly a group of people struggling with similar feelings.

"Could you give me a little time to get used to all this before you contact therapists and groups?" I asked him.

"I can wait a while," Tom said. "But not long."

Even before the obvious signs of maleness, Tom's laughter disappeared from our lives. Overnight, it seemed, he stopped smiling. He no longer took pleasure in anything. He looked ill. He complained of fatigue, stomach ailments and dizziness. He lost his appetite and began to lose weight. But my sincere attempts to sympathise with him alternated with bewilderment and rage over the close, secret relationships he'd apparently formed with women confidantes, over his insistence that his urgent need to express his femininity outweighed every other concern.

"I have a medical condition," he insisted. "A fatal condition that's going to kill me unless I get treatment."

"Who decides the treatment?" I asked.

"I do!"

It was hard to understand the sudden dramatic change in a state of being he now claimed was lifelong. I tried to convince Tom that he was not a woman. When that failed, I tried to convince him that, for our children's sake, he could believe he was a woman and still choose to live as a man.

For his part, Tom's perspective was that if I loved him, I would accept that a transsexual has to do what a transsexual has to do – and sacrifice my own identity accordingly. When he wasn't telling me that the person I thought I had known had never existed at all, he'd say it was a sign of my limitations that I couldn't grasp the idea of same person, different package.

"After all," he said blithely, "the changes I'm making are pretty superficial."

"If they're so superficial, why do you have to turn all our lives upside down for them?"

He didn't seem the same. He didn't act the same. His values seemed to change along with his personality.

"What if you knew that doing this would destroy one or all of the children?" I asked him. Ice cold, the man I had once thought a wonderful father replied, "I would do it anyway."

When I eventually got round to reading other women's accounts – that is, the accounts of women who stayed with their transsexual husbands – they said about their partners what my husband said about himself: he's still the same person inside. "Where inside?" I wanted to shout.

This argument reached an absurd zenith on the day he declared, "You only loved me for my gender!"

"Yes," I said sarcastically. "Since nobody else had that gender, I had no choice but to love you."

It began with a pair of purple cotton underpants. A woman's underpants. I pulled them out of the dryer amid the rest of the usual laundry produced by a man, a woman, two children and one baby. I had never seen them before. Tom came upon me in the basement, standing before the dryer, staring at them.

"Oh, sorry," he said finally. "Did I put those in the laundry? I've been trying to keep them out of your sight."

"That's OK," I whispered.

This was the first time I had ever seen an item of female clothing that belonged to my husband. It was also the end of Tom trying to keep women's clothes out of my sight.

Female clothes – tarty and juvenile, conservative and middle aged – appeared in our home. His new things came from charity shops, where he openly shopped for himself in our small community, and from a growing network of women who saw my closet as the repository for their castoffs. Tom acquired garments from all over the fashion map, ranging from things that I would know weren't mine even if I was struck blind (most) to the occasional item that resembled something I wore. I felt ill handling his women's wear, but sometimes I had to examine the family laundry closely to separate what was his from what was mine.

Tom was allowing his once very short, mostly grey hair to grow out. It looked terrible, but of course that was beside the point. He brought home a hairbrush and kept it in the bathroom closet. One day, he walked into the bathroom while I was combing my hair. He positioned himself next to me before the mirror and began to brush his own hair. When he was done, he smirked knowingly at his reflection and, tracing an exaggerated arc with his hand, very deliberately dropped his brush into a basket I kept for my things beside the sink.

Christine Benvenuto with her youngest child the summer before her husband's revelations. Photograph courtesy of Christine Benvenuto

Such moments packed a breathtaking array of meaning and emotion. All at once there was the pathos of witnessing a middle-aged man – the husband I loved and had admired – taking pleasure in gazing at the woman he evidently saw when he looked at himself in the mirror. His satisfaction with himself. His in-my-face "I'm going to do this and you have no choice but to accept it" attitude towards me. The painful fact that such moments represented his departure from our marriage and from the person he had been, and that I was forced to watch that departure not once but over and over again. The terrible feeling of intrusion into my space, my privacy. Like a rebellious teenager, he wanted me to know: you aren't the only woman around here any more. He wanted me to know: absolutely nothing will be left to you. My basket had become a public receptacle marked All Women's Things Go Here. Like womanhood itself, it was no longer my domain.

Tom found a circle of women to sympathise with, encourage and dress him. Once, he left his laptop open to a message from one of them that read, "Your wife has to accept losing you." He reported that another had urged him to "Do it all quickly!"

From his cheerleaders I learned that in the new political correctness, female solidarity is out. A man in a dress is in. Among women who consider themselves feminists, a man who declares himself a transsexual trumps another woman any day. One of Tom's supporters would eventually sum up this perspective most explicitly: "He's a transsexual. Anything he does is what he needs to do."

These career women told Tom, and some would later tell me, that my wifely role was to support my man and to get my children on board with the project. My responsibility was to Tom. Tom's responsibility was to Tom. In the Valley of the Politically Correct, being a transsexual means never having to say you're sorry.

Tom shaved off the beard he had been wearing since I met him at 17. He shaved off the chest hair I had loved to run my fingers through. One day he came home with his eyebrows plucked to within an inch of their lives, a style choice I tried unsuccessfully to convince him no actual woman had made since the 1940s.

In our joint account I saw payments to a voice coach. I discovered that he carried a portable tape recorder with him during solo drives, so that he could work on raising his pitch. I found this out when he let our toddler play with the tape recorder, a button was hit, and out of the machine came a weird, feminised lisp that neither the children nor I had ever heard before: Daddy's new voice.

Did the kids notice Tom's transformation? They didn't say and I didn't dare ask. Neither the kids nor I would actually see him dressed as a woman during the two years his transformation took place under our roof, or for many months after. We didn't have to confront him modelling the new threads, but I, for one, couldn't forget that they were there.

Knickers that weren't mine were now regulars in our laundry. I also caught glimpses of their lace edges peeking out of his jeans when he bent over to help one of the children, and a bra was sometimes visible underneath his (man's) shirt. He said it made him feel better. Presumably the falsies I found around the house also made him feel better. The only problem was, they made me feel worse. I felt like a woman encountering the presence of an intruder in her marriage in the traces of infidelity among her husband's things. Only the lipstick smears weren't on my husband. They were my husband's.

Again and again Tom promised he would do nothing further; again and again he broke this promise. To my anguished and outraged, "But you said …" he'd tell me, sometimes in anger, sometimes icily cold, "That was yesterday. I didn't say anything about today."

When people ask how I continued to live so long with a man who was no longer my husband, the truest answer I can give is, for my children. Day by day I begged Tom to grant our children a little more childhood. For more than a year and a half, I put off telling them. As anyone who knows kids will guess, this ultimately proved a losing strategy, as their growing awareness that their father was changing and that something had gone terribly awry in their parents' marriage erupted in confusion, fear and stress.

I took it for granted that if Tom was really going to live as a woman, he would move away, or the children and I would move away. It went without saying that I wasn't going to attempt a fresh start in the small town in which we had lived together as a happy family, passing Tom on the street in a dress.

When I put this to Tom, he erupted. "I'm not going anywhere. I'm not leaving this house. I'm going to do what I want to do and I'm going to do it right here."

"But you want to make a fresh start," I spluttered. "We need one, too."

"You're not making a fresh start!" He was furious. "You have no legal right to the house or the kids. They belong to me. If you want to leave, go right ahead. But you're not taking the kids with you."

I was stunned. This was the emergence of the new Tom, one I'd come to know very well over the next several years. The one who intimidated and threatened, who laid down the law and expected me to abide by it. If Tom was becoming a woman, he had never seemed so male – a tyrannical bully he had never been in our marriage.

Many conversations followed from that one, and in this respect Tom remained consistent. The new life, the choices and decisions, were his. The children and I would live with whatever he decided.

Around our town Tom began to wear gender-neutral clothes, which in actual fact meant female but not overtly feminine: women's jeans, a blouse kept zipped inside a navy blue sweatshirt. He went about looking pale and dreadful, and speaking in an exceedingly odd, high-pitched whisper, and so some people concluded that he was ill. I wanted desperately to contain the truth for my sake and my children's. By continuing to live with him, I could at least forestall the day he would appear in full female regalia in front of the children or in our community, because Tom had grudgingly come to realise that, for the time being, forcing me or the children to see him, as he put it, "dressed" would not be wise. Again, my delays were a losing strategy. Tom was not trying out a possible lifestyle. He was making permanent changes. By the end of the first year, his most valuable beauty tool was a daily dose of female hormones.

It is inescapable: for me there is something slightly creepy and more than slightly sad about a man in women's clothes. Male legs in sheer stockings. The sight of Tom in an exact replica of a skirt that was once my favourite. It is creepy for one woman to copycat another, the stuff of thrillers. Creepier for a man to do the same. Creepier still if that man is your husband.

Looking back, I can say Tom was a wonderful husband, father, friend. Or I can say Tom was a fabrication. A fake, who didn't want to be with me, he wanted to be me.

When he moved out, all I was left with was his male wardrobe: a collection of trousers, shirts, jackets and ties. Clothes I had loved and, in some cases, given him. It was as if he had left the bedroom expecting to come back. As if he had suddenly died.

It was a Tom reborn who loaded up his car and said goodbye to his children. This Tom was upbeat and energetic, eager to set off on his new life adventure. He had rented a room in a house in another town, and he would visit the children several afternoons a week. The children ran manically in and out of the house, confused. Their father moving out was a bad thing, right? But he looked so happy!

Tom and I have since divorced. I want to say the kids are all right now. The truth is, some days they are. Some days one or two of them are. Children look to adults to stay the same. Mine have watched their father change his personality, his appearance, his lifestyle, his address and his name. Their experience of Tom's transformation and the break-up of our marriage is hands down the ugliest and most painful aspect of this story.

Recently, out walking, I passed a young family: a mother and a father with a baby in a pack on his back. Watching them together, I was rushed by memories. Tom and me with one, then two, then three small children, babies in backpacks. My God, we were happy! That's what hit me. In recent years I've absorbed Tom's revisions, come to believe I was delusional to think for so long that we were happy. I was not delusional. We were happy. We had a long time together. Now that time is over. We were married and now we're not. My children had a father, now they don't. I can never have complete closure. The man I was married to, the man I loved, no longer exists. But he didn't die. If his death occurred now, it wouldn't be the death of the man I married, but the death of the person he's become. When I think of him in the present tense – for example, when he's on his way over to pick up the children – I unconsciously anticipate the arrival of a person I can more or less recognise. It never happens. When I see him, he is a stranger. A stranger I will never know. I can't do anything about that. Except cease to let it trouble me.

Some names have been changed.

• This is an edited extracted from Sex Changes: A Memoir Of Marriage, Gender And Moving On, by Christine Benvenuto, to be published on 13 November by St Martin's Press.