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A federal judge Tuesday declared Pennsylvania's ban on same-sex marriage unconstitutional, the fourth such ruling on a state ban in the past three weeks.

Judge John Jones III ruled in favor of 23 Pennsylvania residents who challenged the law.

"The issue we resolve today is a divisive one. Some of our citizens are made deeply uncomfortable by the notion of same-sex marriage. However, that same-sex marriage causes discomfort in some does not make its prohibition constitutional," he said.

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The state's Democratic attorney general, Kathleen Kane, announced last July that she would not defend it, saying she could not ethically defend a law she considered "wholly unconstitutional." Defense of the law was carried on by the administration of the state's Republican governor, Tom Corbett.

"Today, in Pennsylvania, the Constitution prevailed. Inequality in any form is unacceptable and it has never stood the test of time," Kane said in a statement. "I have remained steadfast in my decision not to defend Pennsylvania's Defense of Marriage Act because I made a legal determination as to the unconstitutionality of this law."

In declaring Pennsylvania's ban on same-sex marriage unconstitutional, the judge did not issue a stay of his own ruling.

Similar decisions in state and federal courts this month struck down bans in Arkansas, Idaho, and Oregon. Appeals courts stayed the effect of the rulings in Arkansas and Idaho. The Oregon decision is unlikely to be appealed, because the state did not defend the law.

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The ruling in Oregon made it the 18th state to permit same-sex marriage. An effort to appeal the ruling by the National Organization for Marriage, which opposes marriage for gay couples, has been unsuccessful.

With the addition of Pennsylvania, 44 percent of the nation's population lives in a state that permits gay and lesbian couples to marry.

While most states that bar same-sex marriage do so through constitutional amendment, Pennsylvania is one of four to ban it by a state statute. Its law, called the Defense of Marriage Act, was passed by the legislature in 1996 and signed into law by then-Gov. Tom Ridge. It defined marriage as "a civil contract by which one man and one woman take each other for husband and wife" and explicitly barred marriage between members of the same sex.

—Pete Williams