Robin Allen QC, lawyer for the Equality Commission, outlined his case in the second day of an appeal by Ashers Bakery against a discrimination ruling.
Many businesses printed messages they did not associate with, the lawyer said, citing the example of posters made by candidates fighting last week's Assembly election.
Ashers Baking Company in Belfast could decide whether or not to include its logo on its products, Mr Allen said.
The McArthur family, who run Ashers, are seeking to overturn a court judgment which found they acted unlawfully by declining the order placed by LGBT activist Gareth Lee in 2014 based on their religious conviction that the slogan was sinful.
Mr Allen said: "Mr Lee was not asking them to promote anything and in the offering themselves they did not consider themselves to be promoting when they had the icing done on the cake."
He told Belfast's Court of Appeal a flier advertising the service and the fashion in which the order was taken was evidence that it was not a forced act.
"It is no more forced speech than any of the delivery merchants or the post office or any of the companies that printed the numerous hoardings around Belfast and the rest of Northern Ireland for the Assembly elections this week."
The terms and conditions imposed by Ashers did rule out pornographic or blasphemous messages and one of the judges hearing the appeal, Lord Justice Reg Weir, questioned whether the McArthur family may have interpreted the phrase Support Gay Marriage as blasphemy.
That issue has not been raised by lawyers for Ashers.
Attorney general John Larkin QC has been granted permission to take part after arguing that sexual orientation regulations in Northern Ireland discriminate against those like the McArthurs who hold religious or political beliefs.
He said the argument centred on expression: "I say very clearly, if it was a case where Mr Lee had been refused some of Ashers' excellent chocolate eclairs because he was gay or perceived to be gay I would be standing on the other side of the court.
"But it's not about that, it's about expression and whether it's lawful under Northern Ireland constitutional law for Ashers to be forced ... to articulate or express or say a political message which is at variance with their political views and in particular their religious views."
He claimed part of the original judgment was wrong: "It is clear the county court judge has given a greater legal value to the right to express a view supportive of gay marriage than the defendant's wish not to be compelled to express that sentiment and that is simply a failure (involving constitutional law)."