That meant his two mums and his dad.

For Mr Atkinson's whole life his birth certificate has noted only his biological mum Tracey Atkinson.

So when he finished school he felt it was high time his non-biological mum and Ms Atkinson's partner, Alison Todd, and his biological father Peter Todd (Alison's brother) were also represented.

But handing over his application forms, he was told he would have to choose between Alison and Peter. It was a choice the now 25-year-old refuses to make.

"It was gobsmacking and just ridiculous to think it would even be a question, to choose between your parents," he said.

After reading an article about a school's decision to screen a documentary about children with gay parents, Mr Atkinson said enough was enough.

"There are mainstream institutions and media outlets telling 'gayby' children that their parents' love isn't real, it's not normal. I just want to put it in black and white for kids that that is ridiculous."

Mr Atkinson has now launched a Change.org campaign, calling on NSW Attorney-General Gabrielle Upton to allow for changes to the Status of Children Act 1996 (NSW) to reflect that a child might have more than two parents.

Spokespeople for both the Attorney-General and the Registry of Births, Deaths and Marriages said there were circumstances where the parentage could be amended to remove or add a parent. However under legislation, there is provision only for two parents to be listed on a birth certificate.

Family law solicitor Ben O'Sullivan said Mr Atkinson's situation is another example of the law failing to keep up with the community.

"What message do we send to that child who has grown up thinking he had a dad, only to learn later on that at law, in fact that dad is not dad, and worse, the law says he cannot be dad unless one of the mums will no longer be mum.," he said.

"In NSW, The Status of Children Act sets out various parenting presumptions. In the case of the sperm donor to a child conceived to two lesbian mothers, that sperm donor is presumed not to be the child's parent. That is an irrebuttable presumption."

Historically, this presumption was made to encourage male sperm donors at a time when IVF technology was advancing, and to protect them from any financial obligations.

But Mr O'Sullivan said that was now out of step for many children born to same sex couples, where a sperm donor takes on more than that role.

"It therefore seems manifestly prejudicial at law, for the birth record to not be capable in such a case to be updated to reflect a reality in that child's life of two mums and one dad."

Ms Todd and Ms Atkinson had been in a relationship for 14 years before they had Mr Atkinson in 1990.

All three of Mr Atkinson's parents support his campaign, because they see how important it is to him.

"It didn't come to him until he realised he could put one or the other, but not both, and that left him with a dilemma to choose. That did not sit comfortably with him," said Ms Todd, while Ms Atkinson said society needed to catch up with the norms of today.

"I don't think there is the nuclear family anymore. That doesn't really exist, not in our society anyway. Also it's important to carry on that lineage...so further down the track it will be recorded...through the passage of time."

Mr Todd said that the campaign was testament to the role all three parents played in Mr Atkinson's life, particularly through his primary school age years when all three parents lived together.

"The illustration he has included on the campaign is one he did in his first year of school when the kids were asked to draw themselves. Some children chose to draw themselves with skateboards and caps on backwards, he chose to draw himself with three parents."