I did some research on this very same topic a few years ago. I saved my research. I found in the w49 1/15 p.7 this gem...
TO EASE OR NOT TO EASE
...and it has been found that among prison inmates, a full 100 percent engage in this act. As well, an anonymous survey of 144000 catholic priests and nuns was quite revealing. It showed that most of these ones also regularily engaged in this practice. So the question for true Christians is this, "Would I want to be counted in among those who abviously merit God's displeasure?" Certainly not! So let it be the goal of every true Christian to fully refrain from this practice.Remember what 1 Corinthians 6:13 says,
13 Foods for the belly, and the belly for foods; but God will bring both it and them to nothing. Now the body is not for [easing nature], but for the Lord; and the Lord is for the body.Some may ask the question "What if I see another true Christian secretly engaging in this "unclean", unclean of course by it's very nature. Surely every true Christian would be willing to run to the elders immediatly with the "dirt" on their brother's dirt.
More recently,
*** g87 9/8 19-20 How Serious Is Masturbation? ***
Since easing nature is not directly condemned in the Bible, does this mean it is harmless? Absolutely not! Even those not especially concerned with God’s viewpoint feel uneasy about the practice. For instance, Dr. Aaron Hass in his survey of the bodily functions of 625 teenagers reported: “The majority of adolescents who eased nature reported feeling guilty, ashamed, dirty, stupid, embarrassed, or abnormal.” Surely, easing nature is an unclean habit. But since “uncleanness,” according to the Bible, is a term that allows for a wide range of degree of seriousness, easing nature is not to be classed with such serious sins as fornication or other types of gross bodily immorality.—Ephesians 4:19.
However, God realizes that observing the Bible’s prohibitions against gross bodily immorality is not easy. He, therefore, gives advice on how to avoid bodily immorality. He ‘teaches you to benefit yourself.’ (Isaiah 48:17) The principles of his Word indicate that you “benefit yourself” by strongly resisting this unclean habit, primarily because it smells bad . . .