So where did people get this notion that all sexual sins are equally serious? Commonly cited are the following passages from Alma 39: 2-5,13:
2 For thou didst not give so much heed unto my words as did thy brother, among the people of the Zoramites. Now this is what I have against thee; thou didst go on unto boasting in thy strength and thy wisdom.
3 And this is not all, my son. Thou didst do that which was grievous unto me; for thou didst forsake the ministry, and did go over into the land of Siron among the borders of the Lamanites, after the harlot Isabel.
4 Yea, she did steal away the hearts of many; but this was no excuse for thee, my son. Thou shouldst have tended to the ministry wherewith thou wast entrusted.
5 Know ye not, my son, that these things are an abomination in the sight of the Lord; yea, most abominable above all sins save it be the shedding of innocent blood or denying the Holy Ghost?
13 . . . that ye lead away the hearts of no more to do wickedly . . .
A discussion thread on LDS.net shows that Church leaders have also promoted this idea. In this post, a passage from Spencer W. Kimball's book "The Miracle of Forgiveness" is cited. At the beginning of Chapter 5, President Kimball wrote “There are also sins which approach the unforgivable ones in seriousness but seem to come in the category of the forgivable. These are the diabolical crimes of sexual impurity.”
Thus it's understandable why some Church members would believe that all sexual sins are next to murder in severity. But the LDS Church's procedures in dealing with sin differ according to the impact of the sin and sets a prescribed course of repentance according to the attitudes and capacities of the sinner. Cases in point:
-- A bishop is caught committing adultery with the spouse of another ward member. Such a bishop is almost guaranteed a disciplinary council and subsequent excommunication. The need for such a stern response is underscored by the fact that a bishop is not merely responsible for personal compliance, but also for enforcing compliance by others. Adultery by a bishop (or anyone else in a leadership position) compromises the stewardship.
-- A young Aaronic Priesthood holder confesses to occasional masturbation during a worthiness interview with his bishop. No disciplinary council or membership sanction will result, since it is a solitary sin. Most likely the Aaronic Priesthood member's temple privileges will be curtailed, or some other form of informal probation imposed.
Thus the LDS Church clearly varies its disciplinary response according to the impact and gravity of the sin; it does not use a "one size fits all" approach. Nevertheless, as another person on LDS.net makes amply clear, the tools of procreation are sacred in the eyes of our Heavenly Father, which means the misuse of those sacred tools and abilities in principle cannot be negotiated down to be less offensive in the eyes of the Father. While masturbation is far less serious than adultery, we don't want to send the message that it's OK to masturbate; it is not.
All sexual sin involves misuse of the powers of procreation, but not all sexual sin is equally serious.