A gay man who acted as a sperm donor for a lesbian couple was granted full parental rights yesterday to the child he fathered in a landmark ruling by a Scottish court.

Sheriff Laura Duncan ruled that the lesbian couple did not constitute a "family unit" and the 30-year-old man should be given the rights as the father of the 18-month-old boy. She also called for a public debate on the parental rights of same sex couples and said the issue may have to go before parliament.

The judgment has caused concern among some gay rights groups, who say the lesbian couple should have been recognised as a family in the eyes of the law.

The man, a social work assistant who cannot be identified for legal reasons, had gone to Glasgow sheriff court after falling out with the couple over access to the boy. The child's mother wanted her partner to have parental rights.

In her judgment, Sheriff Duncan accused the lesbian couple of secretive behaviour and manipulating the law. She said the man, who had been approached in a nightclub two years ago and asked to donate sperm to the couple, was named on the birth certificate as the father, visited the child frequently and paid alimony. He had been overwhelmed by his feelings at becoming a father and it would be in the baby's best interests to have access to him.

"The pursuer gave the impression of being a thoughtful individual who considered the long-term implications and issues of what he was about to embark upon," she said. "He did not enter the arrangement lightly."

Sheriff Duncan said the lesbian couple did not constitute a family unit and that being raised solely by lesbians may give the boy problems in later life. She said both women wanted to be known as "Mummy" and it could be expected that the child could be mocked by schoolmates for having two mothers.

She acknowledged that this was the first case of its kind to be addressed by a Scottish court and said a public debate should now be held on the issue of same sex parents.

"It may be a matter of public policy to be considered, whether the partner of a homosexual biological mother or father in such circumstances can properly be considered for parental rights. If so, perhaps that is a matter for parliament after public debate."

Sue Robertson, for the Lesbian Mothers Scotland group, said they were waiting for a separate court judgment on the parental rights of a lesbian couple in Scotland and had been taken aback by Sheriff Duncan's findings.

"This really runs very much against all that has been happening in English law," said Ms Robertson. "In England there have been quite a number of cases where partners in lesbian couples have been granted parental rights in relation to the child. In Scotland, although the legislation is similar, there has been no case of that being done."

Alison Clelland, convener of the Scottish Child Law Centre, said the case was complex but could prove helpful in prompting a debate on parental rights for gays and lesbians in Scotland. The case was significant because the sheriff was prepared to say that being a sperm donor was no barrier to claiming parental responsibilities and rights, and also because the lesbian couple was not seen as a family unit for the purposes of parental rights.

"Same sex couples are certainly discriminated against," said Ms Clelland. "A same sex partner is not automatically recognised under the law and this case now focuses the issue and allows us to revisit it."