Flower shop owner Barronelle Stutzman, 68, of Arlene's Flowers in Richland, Wash. said in her countersuit that religious freedom allows her to refuse the sale, the Los Angeles Times reported Tuesday.
The suit was filed on her behalf by a group that opposes same-sex marriage, the Alliance Defending Freedom.
It says Stutzman has hired openly gay employees and routinely designs floral arrangements for gay and lesbian clients.
However, because of her "Christian faith, she cannot as a matter of conscience participate in or facilitate a same-sex wedding," the countersuit said.
Last month, Washington Attorney General Bob Ferguson filed a consumer protection lawsuit against Arlene's Flowers after Robert Ingersoll and Curt Freed were told the shop wouldn't provide flowers for their same-sex nuptials.
The ACLU filed a similar lawsuit alleging violation of the state's anti-discrimination laws.