A prominent gay rights campaigner has announced his support for the McArthur family, who were taken to court for refusing to produce a cake with a pro-gay marriage slogan at their Christian-run bakery.

In an article in the Guardian, Peter Tatchell said he initially "condemned" the McArthurs' decision, but has since changed his mind.

"Much as I wish to defend the gay community, I also want to defend freedom of conscience, expression and religion," he wrote.

The McArthurs were last year ordered to pay £500 in damages to Gareth Lee, who took them to court after they refused to bake a cake decorated with a slogan supporting same-sex marriage. Judge Isobel Brownlie ruled that the company perpetuated "direct discrimination for which there is no justification".

"My finding is that the defendants cancelled this order as they oppose same-sex marriage for the reason that they regard it as sinful and contrary to their genuinely-held religious beliefs," Brownlie told the court.

"The defendants are not a religious organisation. They are conducting a business for profit and, notwithstanding their genuine religious beliefs, there are no exceptions available under the 2006 regulations which apply to this case."

The case has divided opinion over religious freedom, and the McArthurs have stood by their decision not to produce the cake. General manager Daniel McArthur said after the ruling: "We're extremely disappointed with the judgement. We've said from the start that our issue was with the message on the cake, not the customer and we didn't know what the sexual orientation of Mr Lee was, and it wasn't relevant either.

"We've always been happy to serve any customers that come into our shops."

Tatchell said today that while he "profoundly disagree[s] with Ashers' opposition to same-sex love and marriage, and support[s] protests against them", the court was wrong to penalise them. Judge Brownlie's ruling "sets a worrying precedent", he added.

"The law suit against the bakery was well-intended. It sought to challenge homophobia. But it was a step too far," Tatchell said. The bakery refused to produce the cake because of the slogan written on it, not because Lee was gay. "There is no evidence that his sexuality was the reason Ashers declined his order."

Northern Ireland's anti-discrimination laws were not designed to "compel people to promote political ideas with which they disagreed", said Tatchell.

"The judge concluded that service providers are required to facilitate any 'lawful' message, even if they have a conscientious objection. This raises the question: should Muslim printers be obliged to publish cartoons of Mohammed? Or Jewish ones publish the words of a Holocaust denier? Or gay bakers accept orders for cakes with homophobic slurs?"

If the Ashers verdict sets a precedent for future cases, Tatchell warned that it may encourage far-right extremists to "demand that bakeries and other service providers facilitate the promotion of anti-migrant and anti-Muslim opinions. It would leave businesses unable to refuse to decorate cakes or print posters with bigoted messages."

He concluded that it is "an infringement of freedom to require businesses to aid the promotion of ideas to which they conscientiously object. Discrimination against people should be unlawful, but not against ideas."

Frank Cranmer, of the Centre for Law & Religion and Cardiff University, told Christian Today that he is "not quite so sure about the primacy of individual sensibilities" as Tatchell is.

"I fear the answer to his question 'should Muslim printers be obliged to publish cartoons of Mohammed? Or Jewish ones publish the words of a Holocaust denier? Or gay bakers accept orders for cakes with homophobic slurs?' is, very probably, 'yes – always assuming that the image or words in question are ones that can be published legally'," Cranmer said in an email.

"The balance between free speech and hate speech is an incredibly difficult one to strike and I'm not sure we've managed to strike it yet."

He continued: "To turn the matter upside-down, would an atheist baker be justified in refusing to bake a cake for a christening party because s/he disapproved of Christianity? Or would a secular wedding venue be justified in refusing a booking for a Jewish wedding because the owners had a religious or moral objection to Judaism? I don't think so, in either case. That said, I think that Peter Tatchell makes a very valid point, but one to which there's no quick-fix solution – and I'm sure that the Equality Commission for Northern Ireland thought long and hard before supporting Gareth Lee's action in the first place."