(CNN) -- Nine couples filed a lawsuit in Illinois on Wednesday to challenge the constitutionality of a state law that denies same-sex couples the right to marry.
The lawsuit, filed in state court, comes a year after Illinois implemented civil unions for same-sex couples, the American Civil Liberties Union said in a statement.
The nine couples are being represented by lawyers from the ACLU.
A separate lawsuit was filed Wednesday by Lambda Legal on behalf of 16 same-sex couples who are also seeking the freedom to marry, the ACLU statement said.
"The coordinated cases signal the organizations' shared commitment to ensure that the government treats all families fairly," it said.
Across the nation, 29 states have approved constitutional amendments banning same-sex marriage.
Six states and the District of Columbia allow same-sex marriages, while five states -- Illinois among them -- allow civil unions between same-sex couples.
The lead plaintiffs in the ACLU-backed case in Illinois are policewoman Tanya Lazaro and systems analyst Elizabeth "Liz" Matos, of Chicago's Northwest Side, the ACLU said.
The pair, who have two young daughters, have chosen not to enter a civil union.
"Our relationship is not about some legal benefits and protections, but about love for one another," said Lazaro, quoted by the ACLU. "We love each other; we are committed to one another. Anything short of marriage does not recognize that love and commitment."
Many campaigners also reject civil unions as an unacceptable alternative to having the right to marry.
"What defines a marriage is love and commitment, our hopes and dreams for a life with the person we most love in all the world," said John Knight, director of the ACLU of Illinois' LGBT Project.
"Creating civil unions -- a separate, novel and poorly understood status for gay and lesbian couples -- does not honor the devotion of our families, nor fully protect them, but instead sends a powerful message that our families are inadequate and undeserving."
President Barack Obama announced his support of same-sex marriage, a change in his position, on May 9.
Previously, the president had been murky on his personal view of the issue, saying he was simply "evolving" on the issue after having once opposed it, which left many in the lesbian, gay, bisexual and transgender community displeased.
But while he satisfied many on the left by taking a firm stand on same-sex marriage, certain groups --including some black evangelicals and so-called Reagan Democrats -- expressed dismay over his decision.
Obama's endorsement of same-sex marriage came a day after North Carolina voters passed a state constitutional amendment banning legal recognition of such marriages and other types of gay unions.
Voters in Minnesota, Washington, Maine and Maryland will vote on whether to amend their states' constitutions to ban same-sex marriage in November.
CNN's William Mears contributed to this report.