Pornography

Using pornography to satisfy God-given sexual desire is like eating Twinkies to satisfy God-given hunger.

We mistake taste and titillation as being the totality of the purpose for God-given hungers and desires. Junk food may satisfy our cravings but it does not address the intention of hunger which is to nourish our bodies. Pornography is a cheap substitute for intimacy within marriage. It satisfies the craving but it leaves us undernourished sexually.
Sexual desire is intended to bring us together with our spouse and share the unity we have with each other. It is a celebration of intimate relationship. It is a picture of our vulnerabilities before Christ as we share in His intimate love for us. We display ourselves before God, naked and with blemish, and He accepts us and loves us as we are. Every time we join with our spouse, we are celebrating this image.
Pornography leaves us without most of the other benefits of sex. Intimacy, acceptance, protection, bonding, pheromones, Oxytocin, endorphins, etc. God's purpose for the desires he gives us are for our good. It is grace on God's part that he built in these desires. They make it pleasing and easy to do His will. If we can just believe that His way satisfies more fully.

John Piper has an excellent discussion on this very thing.