Jack Phillips, of suburban Denver's Masterpiece Cakeshop, was cited for refusing service in July 2012 to Dave Mullins and Charlie Craig, a same-sex couple married in Massachusetts.
Phillips referred to his personal religious views, opposing same-sex marriage, in declining their business.
"Over and over the courts say, 'No, your religious beliefs, while important, do not trump discrimination laws,'" said the couple's attorney, Amanda Goad.
Phillips' attorney, Nicolle Martin, said creating a wedding cake for the couple would be the same as expressing a message that goes against his beliefs.
"Everyone can agree weddings carry a certain message, spiritual cultural, symbolic, and because it carries a message, under the First Amendment, he (Phillips) has the right to say it or not say it," Martin said.
Administrative Judge Robert Spencer, of Colorado's Civil Rights Commission, heard the case Wednesday and will issue a ruling in several days, KDVR-TV, Denver, reported.