Two gay couples on Tuesday filed a challenge to Missouri's ban on gay marriage.

Kyle Lawson and Evan Dahlgren and Angela Curtis and Shannon McGinty were denied marriage licenses in Kansas City.

The couples are represented by the Missouri chapter of the American Civil Liberties Union (ACLU), whose lawyers argue that the ban violates the equal protection and due process clauses of the Fourteenth Amendment of the U.S. Constitution.

“This discriminatory treatment is subject to heightened scrutiny because it burdens the fundamental right to marry and because it discriminates based on sex and sexual orientation,” the lawsuit states. “But it cannot stand under any level of scrutiny because defendant's refusal to issue marriage licenses to same-sex couples does not rationally further any legitimate government interest. It serves only to disparage and injure same-sex couples and their families.”

“We think it's unconstitutional,” ACLU attorney Tony Rothert told the AP, “and it's time to be challenged in a Missouri court.”

St. Louis officials on Wednesday issued marriage licenses to four gay couples. The following day, Missouri Attorney General Chris Koster sued the city, though he said he personally supports marriage equality.

(Related: St. Louis marries four gay couples to “force the issue” of marriage equality.)

Missouri voters in 2004 overwhelmingly (70%) approved the state's constitutional amendment defining marriage as a heterosexual union.