Indeed, the Bible does not mention masturbation or state whether or not masturbation is a sin. Jesus speaks against having lustful thoughts and then says, “If your right hand causes you to sin, cut it off and throw it away.”
The Bible nowhere explicitly states that masturbation is a sin, but there is no question that the actions that usually lead to masturbation are sinful. Masturbation is nearly always the result of lustful thoughts, inappropriate sexual stimulation, and/or pornography. It is these problems that need to be dealt with. If the sins of lust, immoral thoughts, and pornography are forsaken and overcome, masturbation will become much less of an issue and temptation. Many people struggle with guilt concerning masturbation, when, in reality, they would be far better off repenting of the sins that lead them to masturbate.
Matthew 5:28 says, "That whosoever looketh on a woman to lust after her hath committed adultery." Those who masturbate know that they lust after a woman in their heart, they think of one or two women while masturbating.
Except if done with absolutely no lust, immoral thoughts, pornography, etc.