Long before he ordered a county clerk to jail on Thursday for refusing to issue same-sex marriage licenses, David L. Bunning first drew attention as a federal judge for a ruling on gay rights.

In 2003, he ordered the school system of rural Boyd County, Ky., to allow a student group, the Gay-Straight Alliance, to meet on school grounds. He later oversaw a legal settlement that included anti-harassment sessions for the system’s students.

Judge Bunning, 49, arrived on the bench with what looked like a conservative pedigree. A former federal prosecutor, he was appointed to the court by President George W. Bush. He is a son of former Senator Jim Bunning of Kentucky, a conservative Republican who is a former major league pitcher who was inducted into the Hall of Fame in 1996.

Judge Bunning has shown a willingness to make decisions that put him at odds with conservatives and he is no stranger to fierce controversies — none fiercer than the case of Kim Davis, the Rowan County clerk who says her religious beliefs prohibit her from sanctioning same-sex marriage. When Ms. Davis stopped issuing marriage licenses after the Supreme Court ruling in June that legalized same-sex marriage, Judge Bunning ordered her to resume, and she refused.

On Thursday, Judge Bunning found Ms. Davis in contempt for defying the federal court and ordered her held in jail. “Her good faith belief is simply not a viable defense,” the judge, wearing a bow tie, said from the bench. He noted that, “I myself have genuinely held religious beliefs,” but added that he took an oath of office to uphold the law.

“Mrs. Davis took an oath,” he said “Oaths mean things.” In his personal views, “I think it’s fair to say” the judge is conservative, said Steve Pendery, the judge executive, or chief executive, of Campbell County, Ky., who is a friend of the judge. “For him, I’m sure this is not a political thing, just a matter of the law.”

In fact, the judge’s mother, Mary Bunning, told The Cincinnati Enquirer, “He doesn’t agree with the Supreme Court but has to obey the law.”

By all accounts, the judge does not have the outgoing, combative nature of his father, whose 2004 re-election campaign leaned heavily on his vehement opposition to same-sex marriage.

Jim Bunning was seen in the Senate as willing to irritate members of both parties. He once publicly called his fellow Kentucky Republican, Senator Mitch McConnell, a “control freak.”

But lawyers who have practiced before Judge Bunning and occasionally see him at lawyers’ social functions call him very measured and a man who keeps his opinions to himself.

“Over all, I’d say he’s an excellent judge,” said Todd McMurtry, the president of the Northern Kentucky Bar Association. “He has a reputation for working hard and keeping his docket moving, and you don’t see his personal views.”

Mr. Pendery, who has known Judge Bunning most of his life, said: “He was always a very responsible, serious sort of student and person. He’s outgoing when he needs to be, but there’s a reserve about him.”

Judge Bunning grew up as the youngest of nine children in the northern Kentucky suburbs of Cincinnati, graduating from Newport Central Catholic High School, the University of Kentucky and the university’s law school. For the first decade of his legal career, he worked in the United States Attorney’s Office for the Eastern District of Kentucky.

After the initial resolution of the Boyd County case, Judge Bunning decided against the school system again in 2006, ruling that it could not let students who opposed gay rights opt out of anti-harassment sessions. He was overruled by an appellate court.

In 2007, sitting temporarily on the Sixth Circuit Court of Appeals, he was part of a three-judge panel that unanimously struck down a Michigan law banning the procedure that abortion opponents call partial-birth abortion.

In 2011, he sided with the coal industry and against environmentalists, upholding a federal permit process that made it easier to get permission for “mountaintop removal” mining. The Sixth Circuit later overturned that ruling.