Church manual for bishops says masturbation is unacceptable to God.
Counseling and Interviewing Guide for Bishops (Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 1981), 9
When a young person reaches the age of a teacher in the Aaronic Priesthood, discretion is still required; but your interview regarding his moral status can be more specific, teaching him to respect young ladies as he would expect others to respect his sisters. Again stress cleanliness of thought and also that he should not be engaged in any practices that tend to create unwholesome thoughts. If the maturity of the boy warrants, it is necessary to more directly teach that masturbation is not acceptable in the eyes of our Father in heaven and is not to be practiced by any person, especially one holding the priesthood ofGod.