The Bohan family: Miriam has one legal parent. She wants two: Mummy Josette and Mummy Liz

Miriam Bohan has an unusual wish for her 18th birthday – the inclusion of both her parents' names on her adoption certificate. Right now, the 16-year-old has only one legal parent. Before she becomes an adult in the eyes of the law, she desperately wants her second mother, "Mummy Liz", to be recognised in the eyes of the law as well.

It is an outrage, says Josette Bohan, her only legal parent, who battled a prejudiced and homophobic system a decade ago to adopt Miriam.Gay couples such as Josette and her partner Liz Buck are not legally allowed both to adopt a child. That almost changed last week, except for a coalition of traditionally minded peers who blocked new legislation in the House of Lords.

"I thought it would go through," says Josette, 47. "Miriam said to me, 'If the Government can decide that a car has to have a spare wheel, then why can't I have a spare parent?' That's a very deep thing for a child to say."

Josette and Liz, her partner of 17 years, live in Oxfordshire with Miriam, a second adopted daughter Martha, 10, and their autistic foster daughter, Kimberly, 19. They are a family in every sense of the word.

But while last week's vote has been described as a bitter blow to the gay rights movement, Josette, a stay-at-home mother with cropped grey hair, a winning smile and a jolly laugh, said it was a much more bitter blow for children. "This is not about gay rights or human rights, at all. This is about children's rights. It is about every child's right to have two parents."

If something should ever happen to Josette, then Liz would have to apply to the courts to remain as Miriam and Martha's sole-surviving parent. Her status is not recognised under the law as it stands.

Miriam came to the couple as an abused and mentally troubled foster child when she was just two years old. "We had never planned to be permanent parents," says Josette. "We are both Christians and had a conflict over whether we should become parents."

But it became clear over three years, as the troubled girl finally learnt to trust and love the couple, that Miriam was meant to stay. "We always say that Miriam adopted us," says Josette. "And her birth mother asked us if we would keep her, knowing who we were and what we were."

As a couple, they were not permitted to adopt. But as a single woman Josette could, so she did, raising Miriam with Liz for the past 13 years. "She has come out to be the most wonderful young woman you can imagine. I can say these things because she is at an age where she can say for herself: 'This was fine for me and this was right for me.'"

Five years later Josette adopted a second daughter. The couple had taken in more than 30 foster children, but Miriam told her mothers that she wanted a permanent sibling. After a stressful two-year approval process, Martha, then five years old, came into their home. "The social workers thought she could run rings around adults – and she came and she did run rings around us – but she is 10 now and absolutely brilliant and tremendous."

Josette admitted that their family's living situation might cause her daughters to stand out in school, but the girls have not suffered an overwhelming amount of teasing. "Vicars' children get teased. Tony Blair's children get teased. All kids get teased, but my kids know that they are the most important things in our life. We love them to the point of stupidity, really."

The gay adoption issue is not something she frequently speaks out on. "I am just a parent. I am not a politician," she says.

But just through people becoming familiar with a family such as theirs, Josette believes, they might become more accepting of the idea. "I call it bringing about a whispering change."

Josette will not, however, try to change the minds of people such as Baroness O'Cathain, who led the revolt in the Lords last week. "These children are very needy and we don't have the time. We do have some people say to us that they disagree with how we are living and I say: 'That's fine.' I, too, used to disagree with it."

Now she has no doubts. Her companion, Liz, agrees: "I have absolutely not one iota of regret." She had a hard time accepting the House of Lords judgement but says: "It affects the children much, much more than it does me".

Miriam, now old enough to understand the issue, is proof. "I don't think [the House of Lords] is listening to the adopted kids," she says indignantly. "They don't really consider how it would make a kid feel to have their two parents on their certificate."

Josette still believes the legal change will go through. "Miriam wants a new certificate for her birthday, and I think we might just notch those papers in time," she says with hope and a wry smile.

1954 Josette is born in Kent

1977 Goes to America to study

1979-82 Trains as a nurse in America

1983 Returns to UK and becomes a full-time foster parent

1985 Meets her partner, Liz Buck

1989-92 Daughter Miriam, then aged three, comes to live with the couple as a foster child and is subsequently adopted by Josette

1995 Kimberly, the couple's autistic foster daughter, comes to live with them at age 12

1997-99 Daughter Martha, then five, moves in with the couple as a foster child and is later adopted

2002 House of Lords blocks gay couples from adopting children