morrigan was 18 when she met the other two. Rose and Seng-gye had been married for 20 years and believed, at least in theory, in open marriage. For Morrigan, they were a breath of fresh air. "It was the Eighties, there was disillusion all around,'' she says, "but Seng-gye and Rose hadn't given up hope. We were on the same wavelength. I could talk with them about anything, there were no taboos.''
Rose was equally attracted to Morrigan. "Although I was a free-thinker, I was inhibited emotionally. Morrigan had grown up post-women's- lib and had a confidence I lacked. But being a feminist, she didn't act like a rival. I didn't know about sisterhood before I met her and she became my first close female friend.''
In its time-honoured way, sex was a catalyst for change. Impulsively, one afternoon Morrigan invited Seng-gye to sleep with her and fell wildly "in lust''. Stricken with guilt, she laid low, but Rose caught her out.
"Don't feel guilty, I'm not going to be angry,'' were her words. "I've told you before, I don't own Seng-gye. Now let's sit down and talk about how we are going to work this out.''
"What was important about sex with Seng-gye,'' says Morrigan now, "is how it changed the relationship between the three of us - it gave us a new level of intimacy. When Rose suggested I move in with them, I thought, "Why not?'' I felt between us we could handle it.''
Like all the best marriages, the Tombs-Curtis relationship is founded on a friendship of like-minded people who enjoy each other's company. Unlike most marriages, theirs is polygamous (though only Rose and Seng- gye are legally married). Rose, 44, Morrigan, 26, and Seng-gye, 48, have been together eight years and have three children between them.
As by profession they are writers and researchers (and by necessity canny carpenters), every available space in their five-bedroom house in Bristol is filled with books, files and computers. The children, Brahm, his sister Aditi and brother Hari, are being educated at home and have their own school room.
Downstairs opposite the woodwork room is the communal bedroom with wall- to-wall mattresses and child-size ones nestling at the end. "Like a traditional long house,'' says Seng-gye, who, like the others, is an "anthropology junkie''. Bristol-born and bred, he adopted his Tibetan name.
Their anthropological research reassures them that polygamy has traditional roots. Where they part company from most other cultures is that this is an equal-opportunity household with a matriarchal mind-set. Domestic chores are shared and it's up to the women to initiate love-making - Seng-gye doesn't make any passes.
"There is a lot about being a man in this society which has never felt right to me,'' says Seng-gye. "I'm supposed to be on top, in control, in charge and I've never felt comortable with that. Living with two strong women has mellowed me. I feel more able to relax and be myself.''
Decisions are taken by consensus - experience has shown them that "dictatorship by the majority'' isn't successful. "It may be rambly,'' says Morrigan, "but it's worth it. I prefer an hour's discussion to a five-minute fight which flares up two weeks later because the issues haven't been talked about.''
At first they would spend hours talking, and as the issues were thrashed out, the benefits of a third person who acted as a kind of live-in mediator became apparent, especially for Rose and Seng-gye. "So many arguments in a marriage take place in a war-zone,'' says Seng-gye, "and couples can get locked in adversarial positions. Rose expected me to take the lead but resented me when I did. And I didn't listen well, I didn't know how Rose was really feeling. With Morrigan around, there was someone else to say, 'Look, you think she said that, but really she said that'.''
After eight years, sex is no longer a burning issue, but in the early days they had separate rooms and every night there would be respectful discussions about where Seng-gye was sleeping.
"I'm not saying it was easy,'' says Morrigan. "I felt deeply conflicted that the man I lived with slept with someone else. I didn't think it was wrong but I felt left out, sometimes very isolated. I had stabs of uncertainty about the rightness of it all. What was good was that I didn't have to pretend that everything was fine. When I felt iffy, I could talk about it.''
For a long time now Morrigan has felt so settled and safe that it's hard to recall what jealousy felt like in those early days. "I really believe it's an emotion we've been conditioned to feel,'' she says. "You can unlearn jealousy just as a batterer can unlearn that violent emotional rush when his wife doesn't do what he wants her to.''
For Rose, sexual jealousy was never a problem. "Experience had taught me that sex can be separate from love. What made me jealous was Morrigan's ability to express herself and writing was an area where we were competitive. But now we share skills much more.''
Four months after Morrigan moved in came discovery of her unplanned pregnancy. It clarified their commitment to being a family. Again, one might expect Rose, who had long been trying to become pregnant, to be jealous. "Perhaps if Morrigan had been brimming with confidence, I might have felt shut out,'' she says, "but because she was apprehensive, I felt I had something to offer. Brahm's birth was another stage in our sisterliness, and being able to support her deepened the bond between us.''
About a year later, Rose became pregnant with Aditi. She feels that helping to look after Brahm gave her confidence as a mother and this somehow released her fertility. "The only sadness was that I found out I was pregnant the day that Morrigan miscarried,'' she says. Morrigan then became pregnant with Hari, the youngest child.
The polygamous set-up yields practical benefits, giving the women more time to pursue their careers. As for the children, they called all three adults "Mum'' when they were little - a dad lived in storybooks. Now they are more interested in whose womb they came out of and who breastfed them. They aren't curious yet about Seng-gye's procreative role.
Seng-gye remains enchanted with the community they have created. "It continues to be enriching and stimulating. There are fewer major issues to sort out so there is a more calm and gentle feel. But that doesn't mean there is room for complacency. We are very aware of each other, how each is feeling."
"It could have been such a mess,'' says Morrigan. "I mean, on paper it looks pretty dodgy: young mistress becomes pregnant after moving in with her partner and his wife. But I was young and just assumed that, of course it would work! The fact that it has is a testimony to the effort and responsibility that's gone into it.''
Rose measures how she has changed in the past eight years: "I used to be very angry and I had a severe alcohol problem. But the more I got to know myself, the less I needed to drink or get angry. Living with Morrigan has been such an education. With the support of another woman, I've found the confidence to trust my own feelings and to speak my own thoughts."