God created sex to be intimate. It has an important spiritual quality that is unmatched by other physical functions. To be united sexually in love with a marriage partner is a wonderful thing. The joy of other physical functions, such as eating a good meal for instance, are not even on the same plane with the joy of giving your body to the person who has a legitimate claim on you. Unfortunately, though, this also means that sexual sins are really more serious than some other sins:
Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body. (I Corinthians 6:18 NIV)
Sex is so important because it involves surrendering not only your body, but your soul and desires to another person. In essence, you are giving them your entire identity. When done right, this is a great blessing, but when done foolishly it is a grave mistake. I pray that we may all be good stewards of the body that God has given it, and only use sex to honor Him.