If you want to understand American politics today, starting with the prospects for the 2018 midterm elections, you need to know Jim Domen. By the time he was in middle school, Mr. Domen knew he was different. His attraction to boys confused him. He knew it would shock his parents, born-again Christians.

“I tried to read the Bible, and I prayed to change the sinful desires,” Mr. Domen told a radio interviewer in 2013. He tried dating girls, but that didn’t work either.

When he eventually told his parents, they were “devastated,” he has said. They ordered him to seek treatment from a Christian counselor, but his attractions persisted.

For several years in his 20s, Mr. Domen has said, he had a relationship with a man. After the couple split up, Mr. Domen enrolled as a seminary student at Azusa Pacific University, an evangelical Christian university in Azusa, Calif., and it was there that his life changed at last. He met his future wife; took a job at the California Family Council, an affiliate of Focus on the Family, an organization that promotes “biblical” answers to America’s social problems; and worked toward the passage of Proposition 8, California’s 2008 statewide ballot initiative stating that “only marriage between a man and a woman is valid or recognized in California.”