Barronelle Stutzman, who owns Arlene's Flowers in Richland, Wash., said on the shop's Facebook page Robert Ingersoll and Curt Freed were regular customers but she could not supply flowers for their wedding, The Seattle Times reported Thursday.
"I could not do it because of my relationship with Jesus Christ," she said.
In a lawsuit filed in Benton County Superior Court, state Attorney General Bob Ferguson said Stutzman is in violation of the state's Consumer Protection Act. The state Human Rights Commission would normally deal with such a complaint, under state anti-discrimination law.
JD Bristol, an attorney for the florist, said it is not clear the matter is a case of discrimination and the attorney general has no legal standing to sue.
The American Civil Liberties Union has filed suit against the florist, alleging discriminating on the basis of sexual orientation.
Washington voters gave 54 percent approval in November to a ballot question approving same-sex marriage while exempting religious leaders and organizations from any legal requirement to perform such weddings.
Josh Friedes of the gay rights advocacy organization Equal Rights Washington told the newspaper the issue is not marriage but whether "gays and lesbians can go into a place of business and expect to be served just like everyone else."
The Family Policy Institute of Washington has established a defense fund for Stutzman.