It would be easy to say lust, and that would be true on one level. But the deeper problem is pride and rebellion against God. Every time someone yields to the desire to look at pornography, they are saying, “God, your promises are not enough. Your commands are not sweet to me. Your wisdom means nothing to me. Your way does not satisfy me.” Pornography is enshrining our earthly desires as god in the place of the true God of the Bible who promised to supply all our needs.
If God is not enough for us, pornography will never satisfy us either. I am reminded of Jeremiah 2:13, “My people have committed two sins: They have forsaken me, the spring of living water, and have dug their own cisterns, broken cisterns that cannot hold water.” The first sin is the great one. Whenever we forsake God, whatever we turn to will prove to be a broken cistern. That’s why porn never satisfies. It can’t. Turning to porn is like giving salt to a man dying of thirst. It only makes things worse.
See Create in Me a Clean Heart.