Black said he'll issue the ruling April 14. The civil rights attorneys who filed the February lawsuit did not ask Black to order the state to perform gay marriages, and he did not say he would do so.
Gay marriage is legal in 17 states and the District of Columbia. Federal judges have also struck down bans in Michigan, Utah, Texas, Oklahoma and Virginia, and ordered Kentucky and Tennessee to recognize out-of-state gay marriages, though stays have been issued pending appeals.
Pam and Nicole Yorksmith, a Cincinnati couple who married in California in 2008 and have a 3-year-old son, were among the four couples who filed the lawsuit challenging the gay marriage ban and said Black's comments Friday gave them validation.
"It also validates to our kids that we're bringing into our marriage that their parents are recognized by the state that we live in, and that's extremely important," Pam Yorksmith said. "We're teaching kids of future generations that all families are different and just because our family doesn't look like your family doesn't mean that ours shouldn't be recognized."
Nicole Yorksmith is pregnant through artificial insemination with the couple's second child and is due in June.
The Cincinnati-based legal team asked Black to declare that Ohio's gay marriage ban is "facially unconstitutional, invalid and unenforceable," and indicated that following such a ruling, the window would be open for additional litigation seeking to force the state to allow gay couples to marry in Ohio.
"This is a serious problem at the basic level of human dignity," civil rights attorney Al Gerhardstein told Black during Friday's arguments. "That human dignity is denied by the way Ohio treats same-sex couples. This is central to our whole commitment as a nation to equality."
Dan Tierney, a spokesman for Ohio's attorney general, said the state will appeal Black's order when it comes out but declined to comment further.
Attorneys for the state argued that it's Ohio's sole province to define marriage as between a man and a woman, that the statewide gay marriage ban doesn't violate any fundamental rights, and that attorneys improperly expanded their originally narrow lawsuit.
"Ohio has made its own decision regarding marriage, deciding to preserve the traditional definition," state's attorneys argued in court filings ahead of Friday's hearing.
They argued that striking down the law would "disregard the will of Ohio voters, and undercut the democratic process."
Black didn't say why he made the announcement on his ruling before he issues it. But by stating his intention ahead of his ruling, Black gave time for the state to prepare an appeal that can be filed as soon as he does. The state can also work on asking Black for a stay in his ruling pending appeal.
Gay rights organizations praised Friday's development.
"It's only a matter of time before marriage equality is the law of the land in not just Ohio, but every corner of America," said Chad Griffin, president of the Washington, D.C.-based Human Rights Campaign.
"The court's forthcoming action shines a bright light on the fact that same-sex couples are denied their 14th Amendment guarantee of equal protection," said Ian James of FreedomOhio, a group working to have voters overturn the state's ban in November.
Phil Burress, who chaired the 2004 effort to ban same-sex marriage, did not return a call for comment but has said his group is prepared to fight any ballot initiative to repeal the ban.
The lawsuit originally only asked Black to force Ohio to recognize out-of-state gay marriages on birth certificates. Attorneys later expanded it for a broader and more significant ruling, a move that irked the state's attorneys.
"It could require a sea change in the way numerous government agencies and departments (not parties to this litigation) fulfill their duties," they wrote in court documents, referring to a ripple effect that could encompass Ohio statutes on insurance, mortgages, child guardianship and property.
The lawsuit built on the success of another one also filed by Gerhardstein that sought to force Ohio to recognize out-of-state gay marriages on death certificates.
In December, Black granted that request, saying that Ohio's ban on gay marriage demeans "the dignity of same-sex couples in the eyes of the state and the wider community."
The state appealed that ruling, and the case is pending in the 6th U.S. Circuit Court of Appeals in Cincinnati.
Associated Press writer Ann Sanner in Columbus, Ohio, contributed to this report.