A couple denied a marriage license has filed a lawsuit challenging Colorado's 2006 voter-approved constitutional amendment excluding gay couples from marriage.

According to ABC affiliate 7News, Dr. Rebecca Brinkman and Margaret Burd applied for and were denied a marriage license from the Adams County Clerk's office. The clerk offered the couple a civil unions license instead.

“They rejected this offer because civil unions are more akin to a business relationship than to the expression of dignity, love, respect and commitment that married heterosexual couples enjoy because they are married,” Ralph Ogden, the couple's lawyer, told the outlet.

The women allege in their lawsuit that the state's ban violates the equal protection and due process guarantees of the 14th Amendment of the United States Constitution.

In June, the Supreme Court ruled unconstitutional a portion of the Defense of Marriage Act (DOMA), which led to the federal government recognizing all legal marriages regardless of sexual orientation. However, the federal government does not recognize civil unions.

“[B]ecause Colorado refuses to allow same gender marriages, these federal benefits are denied to members of a civil union,” Ogden explained.

Other states which offered civil unions prior to the Supreme Court's decision – Hawaii, New Jersey, and Illinois – have moved or are debating moving to full marriage. However, Colorado is unique among states with civil unions in that its constitutional amendment blocks lawmakers from considering legalizing gay nuptials.

(Related: Hawaii Senate approves gay marriage bill.)