HARRISBURG — The American Civil Liberties Union said it will file the first known legal challenge today seeking to overturn a state law effectively banning same-sex marriage in Pennsylvania, the only northeastern state that doesn’t allow it or civil unions.

The lawsuit, to be filed in federal court in Harrisburg, also will ask a federal judge to prevent state officials from stopping gay couples from getting married. It names Gov. Tom Corbett, Attorney General Kathleen Kane and three other officials. The plaintiffs are one widow, 10 couples and one of the couples’ two teenage daughters, and they include four couples who were legally married in other states but whose marriages go unrecognized by the state of Pennsylvania.

Pennsylvania would become the 14th state to legalize gay marriage if the lawsuit is successful. It also would force the state to recognize the legal marriages of all same-sex couples in other jurisdictions.

The plaintiffs, including Deb and Susan Whitewood, who have been together for 22 years, said their willingness to join the lawsuit was driven both by a desire to have the same legal and financial protections afforded to opposite-sex couples and the emotional satisfaction of seeking social justice.

“I wanted our relationship to be respected like everybody else’s relationship,” said Susan Whitewood, 49, who lives in the Pittsburgh suburb of Bridgeville. “That was first and foremost the reason for doing this. I wasn’t looking for legal validation.”

Her 16-year-old daughter, Abbey, who is also a plaintiff, said she is excited to fight for her family’s values.

“I want to prove that young people like myself can change the world if they stand up for what they believe in,” she said.

In the lawsuit, the ACLU said banning gay marriage satisfies no legitimate government or child welfare concerns of the state, since Pennsylvania judges routinely grant adoptions to same-sex couples that are viewed as in the best interest of the child.

“It serves only to disparage and injure lesbian and gay couples and their families,” the lawsuit said.

For instance, the suit says, same-sex couples do not have access to a long list of legal and financial protections as do opposite-sex couples.

Those include an inheritance tax exemption for widows; an automatic power of attorney for spouses in health care decisions; damages and legal recourse under workers’ compensation laws for a spouse who dies or is injured in the workplace; assistance programs for same-sex widows and widowers of military personnel and veterans; pension and survivor benefits for widows and widowers of public employees; Family Medical Leave Act provisions; and a spouse’s Social Security retirement benefits.

The lawsuit was not spurred by the U.S. Supreme Court’s three-week-old decision striking down part of the federal government’s anti-gay marriage law that applies only to legally married same-sex couples seeking benefits from the federal government. But it does quote from the decision, saying the barrier to marriage denies same-sex couples “a dignity and status of immense import.”