BRIGHTON, Colo. — A lesbian couple inColorado has filed a lawsuit seeking to overturn the state’s constitutional ban on same-sex marriage.

Dr. Rebecca Brinkman and Margaret Burd filed the lawsuit Wednesday in Adams County District Court after they say they were denied a marriage license earlier the same day, but were offered an application for a civil union instead, reports KMGH-TV.

The lawsuit alleges that Article II, Section 31 of the Colorado Constitution, which states that “only a union between one man and one woman shall be valid and recognized as a marriage,” violates the equal protection and due process guarantees of the 14th Amendment to the United States Constitution.

Brinkman and Burd’s attorneys argue civil unions are inferior to marriage and more like a business relationship. The attorneys say Colorado’s ban is unconstitutional and cite the U.S. Supreme Courtruling from June that invalidated part of the Defense of Marriage Act

Watch a report from KMGH-TV:

Colorado voters outlawed same-sex marriage in 2006. Lawmakers passed the civil union law earlier this year.

Brinkman and Burd have been in a relationship for 34 years.