Mormons in same-sex marriages are considered apostates acting in opposition to the church, and are subject to disciplinary action including possible excommunication, under new policies issued by the Church of Jesus Christ of Latter-day Saints.

The policy changes issued Thursday also state that a child of gay Mormons shouldn’t receive a blessing as an infant, a tradition that sets the child on a path to church membership. A child of gay parents can’t be baptized as a Mormon until he or she reaches 18 years old—and then only...