The Methodist church put the Rev. Frank Schaefer on trial in southeastern Pennsylvania, accusing him of breaking his pastoral vows by presiding over the 2007 ceremony in Massachusetts.
The 13-member jury convicted Schaefer on two charges: That he officiated a gay wedding, and that he showed "disobedience to the order and discipline of the United Methodist Church."
The jury will reconvene Tuesday morning for the penalty phase, where Schaefer faces punishment ranging from a reprimand to losing his ministerial credentials.
"Obviously I'm very saddened. What we're hoping for tomorrow is a light sentence," said Schaefer's son, Tim Schaefer, 29, whose wedding led to the charges.
Testifying in his defense, the 51-year-old pastor said he decided to break church rules out of love for his son. He said he might have lost what he called his "ritual purity" by disobeying the Methodist Book of Discipline, but that he felt he was obeying God's command to minister to everyone.
"I love the United Methodist Church. I've been a minister for almost 20 years and there are so many good things about the United Methodist Church except for that one rule," said Schaefer, of Lebanon.
Schaefer, who pleaded not guilty to the charges, could have avoided the trial if he had agreed to never again perform a same-gender wedding, but he declined because three of his four children are gay.
The nation's largest mainline Protestant denomination accepts gay and lesbian members, but it rejects the practice of homosexuality as "incompatible with Christian teaching."
The church's lawyer, the Rev. Christopher Fisher, told the jury that Schaefer clearly violated the Book of Discipline. He said the complainant, Jon Boger - a member of Schaefer's congregation - was dismayed and shocked when he learned this year about the ceremony.
Fisher used his closing argument to condemn homosexuality as immoral and said Schaefer had no right to break a Methodist law that bans pastors from performing same-sex marriages just because he disagreed with church teaching. He told jurors they were duty-bound to convict.
"You'll give an account for that at the last day, as we all will," he told the jury, to audible gasps from spectators.
Dozens of Schaefer's supporters stood in silent protest as Fisher spoke, then linked hands and sang "We Shall Overcome" after the jury left to begin deliberating.
Boger, the church's sole witness, testified Monday that he felt betrayed when he found out that Schaefer, who had baptized his children and buried his grandparents, had presided over a gay wedding.
"When pastors take the law of the church in their own hand ... it undermines their own credibility as a leader and also undermines the integrity of the church as a whole," Boger said.
"It's his son. He loves his son. In a way I felt bad for him. But he's also shown no remorse or repentance, nor has he apologized to anyone."
When Schaefer chose to hide the marriage from the congregation, Boger said, "It was a lie and a broken covenant."
But Schaefer testified he had informed his superiors of his part in the marriage. He said he kept it from his conservative church's congregation because it would be divisive.
"I did not want to make this a protest about the doctrine of the church. I wasn't trying to be an advocate," Schaefer said. "I just wanted this to be a beautiful family affair, and it was that."
Schaefer faced no discipline until April - less than a month before the church's six-year statute of limitations was set to expire - when Boger filed a complaint.
Schaefer's son came out to his parents at age 17, revealing he had contemplated suicide over his struggle with sexual identity and the church's stance on homosexuality.
"He had heard messages that were hateful from the church, from the culture around him, that told him you're not normal, you're not valid, you're a freak," Schaefer testified.
The pastor said he and his wife told their son he was a "beloved child of God."
Years later, Tim Schaefer asked his father to marry him.
"To say no to his request would have negated all the affirmations I gave him over the years," he said.