The longtime customer, Robert Ingersall, was marrying his partner, Curtis Freed.
Stutzman says she was exercising her First Amendment rights. She had previously sold the couple flowers and knew they were gay, but told them she couldn't provide flowers for their wedding because same-sex marriage was incompatible with her Christian beliefs.
Washington Attorney General Bob Ferguson and the couple sued her, saying she broke state anti-discrimination and consumer protection laws.
In 2015, a Benton County judge said that's not a reason to deny service, but Stutzman's case was heard by the Supreme Court.
Alliance Defending Freedom attorneys who represent Stutzman say they will ask the U.S. Supreme Court to reverse the decision.
The Associated Press contributed to this report.
© 2017 Cox Media Group.