ST. GEORGE, Utah — Woodrow Johnson was 15, and by the rules of the polygamous sect in which his family lived, he had a vice that could condemn them to hell: He liked to watch movies.

When his parents discovered his secret stash of DVDs, including the “Die Hard” series and comedies, they burned them and gave him an ultimatum. Stop watching movies, they said, or leave the family and church for good.

With television and the Internet also banned as wicked, along with short-sleeve shirts — a sign of immodesty — and staring at girls, let alone dating them, Woodrow made the wrenching decision to go. And so 10 months ago, with only a seventh-grade education and a suitcase of clothes, he was thrown into an unfamiliar world he had been taught to fear.

Over the last six years, hundreds of teenage boys have been expelled or felt compelled to leave the polygamous settlement that straddles Colorado City, Ariz., and Hildale, Utah.