My pastor says that pornography is a sin. Why is that? Doesn't everybody indulge in it from time to time?
You are partly correct in your assertion that everybody indulges in pornography from time to time. Many people, truth be told, do. There are many types of pornography. There is video, picture, books, and audio. There are even pornographic, better termed as lustful, thoughts.
Your pastor is also correct that pornography is a sin. This point is made clear in several different places of Scripture. First let us look to Galatians 5:19-21: “Now the deeds of the flesh are evident, which are immorality, impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God.”
As used in the New Testament, we find it having at least four different references: First, the reference to pre-marital sex (see 1 Cor. 7:1-2). Second, it is referenced as a synonym for adultery (see Matthew 19:9). Thirdly, the term is used in a broad sense to refer to all types of unchastity (see 1 Corinthians 6:13,18) and, finally, it is brought up specifically in reference to being a harlot or prostitute in Revelation 2:20-21.
Focus in on the words “immorality, impurity, and sensuality.” First, immorality. The greek word here is porneia. This word covers all forms of sexual impurity; originally meaning to act as a harlot, or to be lustful. In the New Testament alone, porneia refers to four things: first in 1 Corinthians 7:1-2 it refers to pre-marital sex. Next, in Matthew 19:9 it is used to mean the same thing as adultery. Thirdly, in 1 Corinthians 6:13,18 it is used in a broad sense to refer to not being chaste. Lastly, it is used in Revelation 2:20-21 to reference being a harlot or prostitute.
The second term that is used in the Galatians passage in Greek is Akatharsia, or impurity. This word is often used in the sense of saying something is physically dirty, but can also be used to describe someone who has a “dirty mind.”
Thirdly, Aselgia is a term used to describe sensuality. This term refers to undisciplined action that has no respect for a person's rights. Pornography fits all of these in some way, but perhaps Aselgia applies to it the most. To be blunt, pornography is wrong, because not only does it involve lust, which is spoken against throughout the Bible, but it also involves looking at someone strictly as a sexual object, rather than a child of God. To look at someone that way is to shame God, and is having a lack of respect for God's creation. In this way, yes, pornography is sin.
One must keep in mind again, though, that this is not an unforgivable sin. Yes, the first passage mentioned does state that people who engage in it will not inherit the Kingdom of God, but it is important to remember the basics of our faith and realize that if we repent, accept and love God, and welcome the Holy Spirit into our lives, this can be forgiven. Our salvation is not determined by this alone.