A showdown over same sex marriage is looming in New York’s top court after an appellate court ruled yesterday that the state constitution does not guarantee the right of gay couples to marry.

Lawyers for the couples involved in the case have promised they will appeal the decision and anticipated that their suit will join a similar case pending before the state’s Court of Appeals.

The disappointment expressed by litigants and gay rights activists at the news of yesterday’s ruling was softened by the expectation that a definitive decision will soon come from the Court of Appeals.

“We’re disappointed that the decision from the Third Department came out as it did but we’re looking forward to getting to the Court of Appeals and presenting our arguments to that tribunal,” an attorney for the plaintiffs in the case decided yesterday, Sharon Mc-Gowan, said, referring to the midlevel court by its district. “The state has placed a complete bar against same sex couples to marry and there’s no justification for doing so.”

The case, Sylvia Samuels v. The New York State Department of Health, was filed on behalf of 13 same sex couples who were denied marriage licenses. The unanimous decision of the five judges on the appellate panel decides not only the case of the 13 couples but two other similar lawsuits pending before that court as well. The trial court that originally heard the case also ruled against the plaintiffs.

In the 17-page ruling, the appellate court reasoned that allowing same sex marriages would forever change the nature of the institution of marriage.

“We find merit in the defendant’s assertion that this case is not simply about the right to marry the person of one’s choice, but represents a significant expansion into new territory which is, in reality, a redefinition of marriage,” the ruling states.

The ruling said marriage has long been treated as a “fundamental right” by courts, which often link “part of its critical importance to its role in procreation and, thus, to the union of a woman and a man.” Such a change to the institution of marriage might “extract some of the ‘deep roots’ that support its elevation to a fundamental right,” the court ruled.

That reasoning failed to convince at least one of the plaintiffs in the case, Assemblyman Daniel O’Donnell, who said the state should not treat its gay couples differently from heterosexual couples.

“I firmly believe that New York State has an obligation to issue licenses to gay people the same way it issues licenses to straight people,” Mr. O’Donnell said yesterday. “My partner and I have been together for 25 years. All we want is to be treated the same as every other tax-paying citizen in New York State. How we’ll pay for a wedding is a mystery for me, but if we win our case we will get married.”

For Mr. O’Donnell, the compelling issue at stake is that he and his partner, John Banta, are not permitted the same benefits that married couples receive. His list of grievances includes tax breaks he and his partner miss out on because they are gay.

Mr. O’Donnell said he will closely follow the case of the five gay couples in New York City represented in the case Hernandez v. Robles. The Court of Appeals, the state’s highest court, is expected to rule on that case in the coming months. Lawyers in Mr. O’Donnell’s case say the possibility exists that his case will be joined with the one pending before the Court of Appeals.

The claims made in the spate of cases on behalf of gay couples now in the court system all argue similar points, primarily that the equal protection and due process provisions in the state constitution guarantee their right to marry.

Such cases are unlikely to be filed in many states because of state constitutional amendments defining marriage as a heterosexual institution.

Currently, Massachusetts is the only state that marries same sex couples. But earlier this week, the highest court in New Jersey began weighing arguments about whether it should follow Massachusetts’s lead.