When Theresa Volpe and Mercedes Santos sought a marriage license in Illinois earlier this month, the Cook County Clerk's Office worker suggested a civil union instead, the women recalled. But Volpe and Santos already had one. Then, the clerk turned to a colleague. "They want an upgrade," she said.

The clerk's words hit home for Volpe and Santos, who have been together for 20 years, but cannot marry because of an Illinois law that defines marriage as between a man and a woman. "It really emphasized the fact that we got the downgraded version," Santos said.

Last year, Illinois Gov. Pat Quinn signed a bill legalizing civil unions for gay and lesbian couples, but Volpe and Santos said they don't think it's enough. They were among more than two dozen gay and lesbian couples who filed lawsuits on Wednesday against the Cook County Clerk's Office, arguing the state unconstitutionally denied them the right to marry. The two lawsuits were filed by the American Civil Liberties Union of Illinois and Lambda Legal, a law firm dedicated to protecting lesbian, gay, bisexual and transgender people.

Volpe and Santos, who have two young children, said they hope the lawsuits will eventually allow them to marry in Illinois. Although a civil union provides the same legal protections as same-sex marriage, they said, is not a substitute. "Marriage means never having to explain that you are just like everyone else," Santos said. "My kids have to explain our relationship all the time and they shouldn't have to."

The Illinois legislature is considering a measure that would overturn the state's ban on same-sex marriage, but it likely will not be voted on before the legislative session is scheduled to end this week. Six states and the District of Columbia have legalized same-sex marriage, either through legislation or court decisions. In 2009, the Iowa Supreme Court legalized same-sex marriage, following a lawsuit brought by Lambda Legal. In California, similar suits led to a state Supreme Court ruling that declared the ban on same-sex marriage unconstitutional. The state's voters approved Proposition 8 in 2008, restoring the ban on same-sex marriage.

The Illinois suits come just weeks after President Barack Obama and Illinois Gov. Quinn voiced support for same-sex marriage. The Illinois ACLU has for more than a decade been in contact with same-sex couples in the state who wish to marry. John Knight, the director of the Lesbian, Gay, Bisexual, Transgender Project of the ACLU of Illinois, said he thinks that the timing for the lawsuits is right -- given rapidly shifting public sentiment and the support of powerful elected officials -- and that the impact of a successful suit in Illinois could be powerful. "Wins in big, important states like Illinois are huge for this country," King said.

Peter Breen, executive director of the Thomas More Society, a law firm that advocates against same-sex marriage, said his organization plans to support the defense of the lawsuits. "Going to the courts to strike down Illinois's longstanding law that declares marriage between one man and one woman is thwarting the will of the people, and doing an end run around the legislature," he said.

Cook County Clerk David Orr said in a statement that the time was "long past due" for Illinois to allow county clerks to issue marriage licenses to same-sex couples.

"In Cook County alone, we have issued more than 2,500 civil union licenses since June 1, 2011," Cook wrote in a statement to The Huffington Post. "This outpouring is a testament to the thousands of families who are denied legal protections, which opposite-sex couples take for granted."

It wasn't immediately clear whether the state will fight the lawsuits. The governor's office did not respond to request for comment, and the Cook County State's Attorney's Office declined to comment. The Illinois Attorney General's Office declined to comment, but said it is reviewing the filings. A Cook County Clerk's Office spokeswoman suggested the clerk, David Orr, would not oppose legalizing same-sex marriage.

"We'd really like the outcome of the lawsuit to be our ability to issue marriage licenses regardless of gender or sexual orientation," said Gail Siegel, the spokeswoman.

CORRECTION: John Knight, not John King, is the director of the Lesbian, Gay, Bisexual, Transgender Project of the ACLU of Illinois.