Although the Bible has much to say about God's plan for human sexuality, it doesn't mention viewing pictures or videos for the intent of becoming sexually stimulated. Still, the English word "sin" is used for several Hebrew and Greek words in the Bible, and pornography matches many of them.
Is pornography a sin?
'awon: The Hebrew for iniquity also infers a perversion of life, truth, and intent. Pornography certainly fits this. The pictures are a perversion in that they take God's intent (intimate sexual relations within a marriage) and pervert or twist that intent. The Bible calls men to "rejoice in the wife of your youth" (Proverbs 5:18), not in images of the bodies of strangers. It also twists the way we see each other. We are to relate to each other as brothers and sisters, in all purity, not as tools for our own gratification (1 Timothy 5:1-2).
rasha': This Hebrew word connotes turbulence and restlessness or something ill-regulated. Repeated exposure to pornography creates a loss of control. It creates an addiction and a desperate need for more. We are called to not be mastered by anything in the world (1 Corinthians 6:12). We are to be filled with the Holy Spirit, which is in direct opposition to the desires of the flesh (Galatians 5:16-17).
'amal: 'Amal is the suffering and trouble the sin brings to sinners or those we sin against. Pornography is not a victimless diversion. Exposure to pornography deeply wounds those who indulge in it. It damages natural physiological reactions, drawing people in to more and more images until the addiction perverts normal relationships. And it damages those who participate, not only because it celebrates the direct defiance of God's plan for women (1 Timothy 2:9), but also because many of those who participate in the making of pornography do so because of manipulation, threats, and violent repercussions if they refuse.
hamartia: This Greek word is used throughout the New Testament. It's often described as "missing the mark" but can also mean a governing principle or power that leads a group to miss the mark. Pornography feeds this power. It feeds the lies that the people in the images are willing participants of the actions of the viewer. It also feeds the lie that indulging in activities that contradict God's plan for us is a physiological necessity. First Corinthians 10:13 disagrees, stating, "No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it." The belief that pornography is a necessity is a lie. God did not design us to rely on a sin.
Pornography—viewing it and participating in it—is sin. It is a perversion of the blessing God intended for mankind. It damages our relationships with God and with others. Acknowledging this is important, but it isn't enough. The sexual liberality of Western culture is a gateway for pornography, and it is largely inescapable. We need to make a concerted effort to minimize the temptation. Avoiding pornography is an obvious step, as is avoiding certain movies and TV shows, and using blockers on computers. As the Creator, God has the power to rescue us from the damage sin causes. Pornography is a tool used by the enemy to destroy the goodness God created. It will take the power of God to conquer it (Ephesians 6:12-13).
Is masturbation a sin according to the Bible?
How do I control my thoughts?
How do I get control of sinful impulses?
Does my private, personal sin affect others?
How can I have victory in overcoming sin?
Truth about Sin