An activist who asked a Christian bakery to make a cake bearing a pro-gay marriage slogan did nothing wrong, his lawyer told the Supreme Court.
The request at an Ashers branch in Belfast was within the firm's ordinary course of business, Robin Allen QC said, but was refused on grounds of religious conscience, prompting a mammoth legal battle.
Another lawyer, for Ashers' owners the McArthur family, told the UK's highest court the state was penalising them after they were found to have discriminated by refusing in 2014 to make the product iced with the slogan Support Gay Marriage.
The order was placed by gay rights activist Gareth Lee.
His counsel Mr Allen said: "What Mr Lee asked them to do was not egregious, not outside their ordinary course of business, it was not a trial and it was not an abuse.
"It is not the case that Mr Lee was doing anything wrong or unusual or strange in bringing in that logo and asking for it to be iced.
"He would not have known that it was outside the ordinary course of business for Ashers."
Outside court bakery owners Daniel and Amy McArthur warned the law risked "extinguishing" their consciences.
Their lawyer David Scoffield QC said: "The notion that Christians may exercise their faith on Sundays but forget about it when they step into work on Monday is not real freedom of religion and is not freedom of conscience."
He said it was a case of forced or compelled speech on the state's part.
"Mr and Mrs McArthur have been penalised by the state in the form of the judgment at the County Court for failing through their family company to create and provide a product bearing an explicit slogan 'Support Gay Marriage' to which they have a genuine objection in conscience."
The court, sitting in Belfast, heard the start of the two-day case on Tuesday. The five judges were Lord Hodge, Lord Mance, Lady Hale, Lord Kerr and Lady Black.
The Ashers legal battle has run for four years and attracted enormous attention.
The original case against the McArthurs was taken by Mr Lee with the support of Northern Ireland's Equality Commission and the bakers lost in the lower courts.
Northern Ireland is the only part of the UK or Ireland where same-sex marriage is outlawed, with Theresa May's DUP allies staunch opponents of the practice.
Controversy first flared when Mr Lee, a member of the LGBT advocacy group Queer Space, requested a cake featuring Sesame Street puppets Bert and Ernie for a private function marking International Day Against Homophobia.
His order was accepted and he paid in full but, two days later, the company called to say it could not proceed due to the message requested.
Mr Scoffield said the McArthur family were being forced to use their skills, trade and experience for a purpose inconsistent with their beliefs and claimed they must: "Choose between operating their businesses or living and acting in accordance with their religious beliefs and we say that cannot be the law."