I have a suspicion that many teachings regarding masturbation are based more on cultural taboos than the Gospel of Jesus Christ. I am unaware of any real scriptural basis for what we (though, really, almost exclusively the youth) are often taught. I also see many surprising parallels with other false notions that spread throughout the church.
When I was initially studying the matter of the Right Hand Rule I was impressed with the solid lack of scriptural basis for the rule. I fully expected to find at least some mention of what hand people were using during the sacrament. Right hands are often prominently mentioned, so I believed that at least this much might be notable. However, there was nothing to be found. I then asked my mission president, and he couldn't cite a source, instead referring to the "unwritten order of things". That was pure lunacy. If it was a part of the basic ordinance, it wouldn't be unwritten. Ordinances do not get communicated in the rumor mills, or by any other indirect and hazy method. They are revealed, and all essential details are clearly communicated to anyone who needs to know them. In particular, since new members need to be taught the commandments before they are baptized, it seemed particularly damning to this doctrine, that it was not included in any discussion.
The suggestion that masturbation is not a serious sin might have some merit. Some prophets have asserted otherwise, (ie. they have asserted that it is serious) but they have done so amidst other patently false assertions regarding the dangers of masturbation.
When put in the position of determining what to teach, I am honestly disinclined to teach that the church has position X, if my belief in that assertion is based on nothing but individual statements that the church does not even bother to document in teaching material, scripture, or policy. It certainly is something I was taught and believed but the body of teaching that convinced me of this was largely no more authoritative and meaningful than my mission president asserting that the sacrament must be taken with the right hand. The For the Strength of Youth pamphlet has dropped the assertion.
It is certainly true, that masturbation increases the sexual appetite, as has been well documented. Therein lies a certain danger. Too frequent masturbation can also have other physically harmful effects.
Masturbation is often conflated with pornography, even in supposedly serious research. Masturbation paired with pornography is certainly a problem, and there is some evidence that some unhealthy fantasies (ie. some paraphilias) can be strengthened and maintained via masturbation.
I am also inclined to think that masturbation, undertaken for the intent of pleasing one's self so that sexual intimacy can more easily be denied to one's spouse, is also sinful.
Overall, according to my current thinking, the evidence would suggest that avoiding masturbation can be beneficial, but that the fear of the practice is largely based on exaggeration and rumor. ie. Maybe not completely a false doctrine, but one which has been greatly exaggerated. My current stance is that masturbation ought not to be forbidden or discouraged except in some limited circumstances.