How does unbelief contribute to the need I feel for pornography?
Feeling the need for sexual titillation is like getting hungry when you haven't had anything to eat for a day. It's as natural as the day is long.
Sex is a desire that rises up within the body and is there. We're just wired for it, men and women (women differently, but not without it). And it wasn't belief or unbelief that made that desire happen. It was hormones that made that happen.
So the first answer to the question, "How does unbelief relate to my sense of need?" is that it doesn't have anything to do with the sense of want or desire. Those desires are just there like your hunger for a steak or your tiredness and desire for sleep.
Now when the question is, What do you do with it?, there unbelief has everything to do with whether you need pornography. I said "titillation" earlier, referring to some kind of sexual feeling—that's natural. But now, if you go towards pornography to do that, that is an issue of sin.
Because pornography is the abuse of women, first of all. And, secondly, it is the illegitimate stoking of a legitimate desire. And God has other ways for that to be satisfied.
Unbelief relates like this: my understanding of faith is that you don't just believe that Jesus as the Son of God died for your sins, rose again, reigns in heaven, is coming again, and forgives my sin. That's only factual. It's also a being satisfied with that, treasuring that. Faith is not just an intellectual assent to doctrines. Faith is an affectional embrace of the Savior for my deepest longings.
Unbelief, on the other hand, is a failure to be satisfied in Jesus. It's a failure to go to him as the living water and the bread from heaven and the light of the world. It's a failure to go to him as a satisfaction that's deep enough and strong enough to satisfy me when I am tempted to go in a sinful direction to indulge an appetite—say, an appetite for companionship or food or sex. The satisfaction of Jesus—that is, belief in Jesus, embracing Jesus, loving Jesus, being content in Jesus—is going to be the power that severs the root of that impulse.
I believe with all my heart that the main way we're sanctified in life is that we conquer the power of the promises of sin with the power of a superior promise, such as Jesus saying, "I'll be with you. I'll love you. I'll take care of you. I'll reward you. You'll know a fellowship with me if you conquer this that you wouldn't have known otherwise."
And who of us has not tasted that, right? We've either yielded and gone into some sexual thing that has made us feel yucky and dirty, and we have to repent and hope we won't do it again; or we've conquered. We've walked into some path of light and holiness and purity, and everything in us is saying, "Yes! O, that I could do that every single time, because it is so much richer and fuller and deeper than the other way!"
If belief includes an affectional embrace of Christ as your deepest satisfaction, then it's going to have a huge effect on whether you embrace pornography.