Jack Phillips from Masterpiece Cakeshop has opened a case at the US Supreme Court.
He was told in 2014 by the Colorado Civil Rights Commission that it was unlawful to refuse the work and that he should re-educate his staff.
Two years earlier he had declined an order claiming his faith meant he could not design a cake promoting same sex marriage.
He's now fighting his case at the US Supreme Court.
Acting Solicitor General Jeff Wall, acting for President Trump, filed papers at the Supreme Court defending Phillips.
In them, he warned a decision against Phillips could create an "intrusion" against the First Amendment "where a public accommodations law compels someone to create expression for a particular person or entity and to participate, literally or figuratively, in a ceremony or other expressive event."
Mr Phillips is being defending by religious freedom charity Alliance Defending Freedom. It says the government cannot coerce Phillips to create artistic expression that communicates a message with which he fundamentally disagrees.
"Nobody should be forced to choose between their profession and their faith," said Kristen Waggoner, Senior Counsel for ADF.
"Phillips gladly serves anyone who walks into his store, but, as is customary practice for many artists, he declines opportunities to design for a variety of events and messages that conflict with his deeply held beliefs.
"In this case, Jack told the couple suing him he'd sell them anything in the store but just couldn't design a custom cake celebrating their wedding because of his Christian faith."