The Bible is not silent on the subject of masturbation. It does not leave us guessing. It's true that Scripture never mentions masturbation specifically. However, because the Bible does speak thoroughly and explicitly about sexuality and sinful lust, it doesn't have to speak explicitly about something so closely related as masturbation.
Let's look at two ways we can know that the Bible speaks to masturbation without ever naming it.
First, consider that if masturbation is extremely common (as are most sins) and nearly always associated with sinful lust, we can safely assume the same was true in the ancient world. So think of Jesus delivering the Sermon on the Mount. He essentially said to imagine having sex with a woman is a kind of adultery (Matt. 5:28). Don't you think masturbation is a clear application and exactly the kind of action He was thinking about?
Second, consider that the Bible never refers directly to abortion. Yet because Scripture speaks clearly about the value of human life and the sin of murder, we are right to conclude that abortion is sin. In almost precisely the same way, because Scripture speaks clearly about the power of sexuality and the sin of lust, we can conclude that masturbation is nearly always sinful. In each case the specific action is so closely linked to the larger category of sin that the connection and shared moral status are simply obvious.
The Damage Done
Why, exactly, is masturbation sinful? Most importantly, just like any other sin, because it violates God's holiness. Masturbation is against God, against His ways and His purposes for how men and women are to relate to one another in a marital union that reflects the relationship of Christ to the Church.
Masturbation is also sinful because it compromises us. We are made in God's image. We are meant to glorify Him in every aspect of our lives, and masturbation hinders us in this mission in two principal ways—by polluting our minds and by inclining us to isolation.
Sexual gratification, of course, is not merely a physical act, but one that engages the mind, often quite intensely. During masturbation, pornographic images, whether seen externally or visualized internally or just plain imagined, nearly always provide a kind of fuel. Indeed, the vast majority of the time these fantasies are nearly impossible to separate from the masturbation itself. This type of fantasy can be dangerous in at least two ways.
First, as most adults have learned the hard way, reality is rarely as wonderful as fantasy. Many people create expectations for sex that reality cannot meet. In fantasy, everything always works, the other person is always willing and able to participate. In other words, it is nothing like real life. And in that way fantasy eventually and inevitably forms unhealthy and unrealistic expectations of sex.
Second, just as sex scenes in movies rarely involve married couples who can, before God, legitimately enjoy sex, fantasy will rarely revolve around legitimate sexual partners. In theory, it is perfectly fine for a woman to dream of a sexual encounter with her husband, but beyond that God gives us no right to fantasize, even about a pretend husband or a person who may one day be a husband. Masturbation, even under those circumstances, may encourage any woman to fill her mind and desires and fantasies with thoughts of other men. And a single Christian woman, having no God-given partner with whom she can consummate sexual desire, simply has no legitimate reason for pursuing sexual fantasy at all.
Some will protest that when they masturbate it is merely a physical act, something done to relieve stress or boredom. They will insist that they do not succumb to thinking inappropriate thoughts. I am extremely skeptical of these claims, but I do not dismiss them, because I cannot see into anyone else's heart or read anyone else's mind. But even assuming, for the sake of argument, that a small proportion of women masturbate without any pornographic images or fantasies in their heads, there is still at least one powerful reason why masturbation is so harmful . . .
A close examination of the Bible's teaching on sexuality uncovers no reason to believe that God ever intended sex to be a private pursuit. Indeed, the heart and soul of sexuality is the giving and receiving of sexual pleasure between two people—one husband and one wife. Sex is intended to be a means of mutual fulfillment, an expression of love in which a husband thinks foremost of his wife and the wife thinks foremost of her husband. It is a uniquely powerful means by which husband and wife can fulfill the Lord's command to esteem another higher than oneself. As they fulfill each other's needs, they also have their own needs fulfilled. It is a beautiful picture of intimacy! As any married couple can testify, the more selfless the sex, the better sex becomes. The more each spouse seeks to please the other, the more fulfilling, gratifying, and beautiful the experience.
This mutual giving and receiving, the heart of God's purpose for sexuality, is exactly what masturbation does not and cannot provide. Masturbation strips sexuality of its divine purpose of mutual fulfillment. Where legitimate sexual expression is meant to produce unity, masturbation produces isolation and division. Masturbation is inherently self-centered. An act meant to be shared toward two people is completely and exclusively about one person, all alone. Masturbation deeply undermines a woman's ability to deny and resist her most self-centered, sinful, isolationist tendencies.
Masturbation simply cannot fulfill God's design for sexuality, and thus has no place in the life of one who calls herself a Christian.