Why does Jeannette’s mother say she can’t leave Jeannette’s father?

Jeannette's mother says that she cannot leave Rex because it is against the Catholic faith.

Mom turned quiet. She seemed to be thinking. Then she looked up. She was smiling serenely. "I can't leave your father," she said. "It's against the Catholic faith." Then she sighed. "And anyway, you know your mom. I'm an excitement addict."