"Nearly a decade ago when the constitutional amendment passed we knew that it violated the equal protection clause," PROMO Deputy Director Stephanie Perkins said.
Now the amendment is being challenged by an ACLU lawsuit.
"We know when the people of Missouri passed the marriage amendment they did not intend to harm families, but the laws of the state do harm them and we need to fix that," Executive Director of the ACLU of Missouri Jeffrey Mittman said.
Ashley and Kate Quinn of Springfield are among eight plaintiff couples.
They were married in Massachusetts in 2010, but live in Missouri where their marriage is not recognized.
"Just to know that we're not alone, that there are other people going through this same experience as well. I feel proud to be a part of it," Ashley Quinn said.
The suit claims the amendment harms these couples by denying them the same rights opposite sex married couples enjoy.
"Marriage is marriage. These couples have made a commitment to each other and it deserves to be recognized," Mittman said.
Right now, same-sex married couples are denied many healthcare and employment benefits.
"I know that when I go to sign Kate up on my insurance at work, I can't. So that's again another added complication," Ashley Quinn said.
By being a part of this lawsuit, the Quinns feel they are standing up for an entire population of Missourians.
"We're really participating in this on behalf of everybody who doesn't have the opportunity," Kate Quinn said.
They hope this is one more step forward in the equal protection movement in Missouri.
"I'd like to see employment protection, housing protection statewide, so that you know people are treated like people and nobody is discriminated against for any reason," Ashley Quinn said.
Similar lawsuits have been filed across the country over the last few years.
Wednesday a court in Kentucky ruled that that state's same-sex marriage ban is unconstitutional.
Copyright 2015 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.