The suits, filed against the clerk of Cook County by the gay rights group Lambda Legal and the American Civil Liberties Union, claim Illinois' refusal to issue marriage licenses to gay and lesbian couples violates the equal protection and due process clauses of the state's constitution, the Chicago Tribune reported.
Supporters of same-sex marriage say they'll keep pressing lawmakers to legalize same-sex marriage -- a House bill introduced this year was pulled during the legislative session -- but the lawsuits bring the issue to the judicial system and could lead to the Illinois Supreme Court.
"We always thought this was something that had to happen. We think it's time to try in the courts and we're optimistic about our chances," ACLU attorney John Knight said.
Illinois Gov. Pat Quinn released a statement reiterating his support for same-sex marriage.
The suits were filed on behalf of 25 Illinois couples who tried to get marriage licenses in May from the Cook County clerk's office but were denied based on a state law barring marriage between two people of the same sex.
Cook County Clerk David welcomed the legal challenge of the state law
"The time is long past due for the state of Illinois to allow county clerks to issue marriage licenses to couples who want to make their commitment," Orr, who is out of the country, said in a statement issued by his office. "I hope these lawsuits are the last hurdle to achieving equal marriage rights for all."
Peter Breen, executive director and legal counsel of the Chicago-based Thomas More Society, which opposes gay marriage, said he expected the state's attorney and attorney general's office to "forcefully defend the state's marriage law."
"We will provide whatever assistance we can to help them in that defense," Breen added.
Robert Gilligan, executive director of the Catholic Conference of Illinois, said the lawsuits prove gay rights supporters who fought for civil unions never planned to settle for that and "same-sex marriage was really the issue."
Six states and the District of Columbia allow same-sex marriage.