Sex is a reflection of what it means to be made in the image of God. God saw that it was not good for man to be alone. We were made for relationships. The creation story shows that intimacy with others is essential to our humanness. There is a sense in which we only fully reflect the image of God when we are in significant relationships with others.
Sex is the highest form of physical intimacy between two people.
Somehow, sex taps into the very heart of our personhood. Sexual immorality defiles an ultimate expression of His image in us. A lack of sexual fulfillment is frustrating for reasons beyond just an unmet physical desire. It causes frustration because it can feel like rejection of me as a person. Also, because I am made in God's image, I have a powerful need for physical intimacy with my spouse.