The most difficult aspect of this issue is that the Bible nowhere even mentions masturbation, and, thus, does not specifically state whether or not it is a sin. This leads some to conclude that masturbation is not a sin. However, the absence of a clear statement in the Bible does not necessarily indicate an act is not sinful. At the same time, the lack of a clear "masturbation is a sin" statement also makes it impossible to give an explicitly biblical argument that masturbation is a sinful act.
Is masturbation a sin according to the Bible?
The most commonly used scriptural passage to point to masturbation is Genesis 38:9-10. Interesting, in several languages, the word for masturbation is a variation of the name "Onan," the individual spoken of in Genesis 38:9-10. However, the passage is not speaking about masturbation at all. Rather, the passage is speaking of a brother refusing to fulfill his duty to produce an heir for the line of his deceased brother. Matthew 5:27-30 is another passage sometimes used to prove that masturbation is a sin. While the parallels between having lustful thoughts, sinning with your right hand, and masturbation, are clear, it is reading too much into the passage to make Jesus' words directed towards the issue of masturbation.
When the Bible does not specifically deal with an issue, biblical principles must be applied. With the question of whether or not masturbation is a sin, several principles definitely relate. (1) We are to avoid lustful/immoral thoughts (Matthew 5:27). (2) There is not to be even a hint of sexual impurity in our lives (Ephesians 5:3). (3) We should not do anything that we cannot do for the glory of God (1 Corinthians 10:31). (4) We should not do anything that we are not fully convinced is good and right (Romans 14:23). (5) Our bodies belong to God, not to us (1 Corinthians 6:19-20). If an action is not in agreement with the above principles, it should not be done.
What is most interesting about the "is masturbation a sin" question is the fact that more attention is given to the act itself (which the Bible does not explicitly say is a sin), rather than the things that almost always lead to masturbation (which the Bible does declare to be sinful). The sins of lust, immoral thoughts, and pornography are what almost always cause the temptation/urge to masturbate. Rather than focusing on the final act, so to speak, more attention should be paid to overcoming the sins that lead to masturbation. Whether or not masturbation itself is a sin, and whether or not other sinful acts were involved, masturbation is not an unforgivable sin, nor should it lead to debilitating guilt and shame. Rather, just like all other sins, it should be confessed to God (1 John 1:9) and wholeheartedly turned from.
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Truth about Sin