Anyone who has any understanding of the Bible knows that God has limited the sex relationship to one man and one woman within marriage. Why did He do that?
Our world seems to have gone sex-mad. Christians of every age are influenced to immorality. The temptation is strong. Many Christians know God has said "No" but they have no idea why. Of course, obedience doesn't need reasons. The fact that God has said, "In marriage only" is enough; however, reasons help us to defend our faith and show the wisdom of God.
As with all things, God is the center. God designed the marriage relationship, including intimate relationship, as a picture of the tender care He has for the Church in general and Christians as individuals. It portrays the faithfulness, commitment, and single-mindedness of God to His people, i.e. Christians. It is also expresses the intense joy, fulfillment, and satisfaction that is to be found in God alone.
Immorality, then gives false testimony to the character of God. It pictures Him as a selfish Being using and abusing people for His own pleasure. It says God is unfaithful and not to be trusted. It is the teaching of false religions which proclaim that "all paths lead to God." God does not flirt or go after others. There is only one way to God - Jesus Christ. Satan delights in immorality for it slanders Who God is.
Marriage is for two people of the opposite gender who are united in God for a purpose. Some feel that having children is the main reason for marriage. It is an important purpose, true, but it is not the only purpose. God has a purpose for every couple. They are to be focused on the purposes of God to expand the Kingdom of God. Sex is an expression of that unity. Intimacy outside of the marriage breaks that unity and destroys the purpose of God for the couple. It is one of Satan's ways of slowing down the expansion the Kingdom of God.
Side note: One of the important reasons why people of all ages give in to temptations, suffer from depression, and/or are "bored" is because they have not discovered God's purpose for them in life. People with purpose are better able to resist temptation and live happy, healthy lives.
Keeping intimacy in marriage teaches self-control. Being able to control ones actions and focus them is one of the keys to successful living. Controlling one's sexual drive and focusing it on one's spouse alone teaches control in every area of life. People with no or little self-control self-destruct. Unfortunately, they not only hurt themselves, but also much damage is done to many innocent lives.
Limiting physical intimacy to marriage leads to unselfish maturity. People who are immoral are seeking only to satisfy themselves. Intimacy in marriage is intended to teach the partners on how to focus on each other. They are to seek the benefit and fulfillment of the other person above their own. It is an important way of seeking to bring the other person to the highest and best God has for them. This is not to say that all marriages act like this. There are selfish, immature marriages. What we are looking at is the ideal that God has for us. The sexual relationship may not reach the ideal within marriage, but it will never reach the ideal outside of marriage.
Keeping intimacy within marriage creates the potential to provide a strong and stable home in which children can grow up. God loves children and knows they need a stable environment to feel secure and to mature. When they have confidence that Mom and Dad love and are committed to each other, then they can face the storms in their lives from a firm foundation. Immorality breaks this security and introduces fear and instability.
The family unit provides a training center for children from the moment they are born. It teaches not only by words but also by example. Children need to see the example of father and mother totally committed to loving and supporting each other. It pictures to the child the committed loving care of God. God's desire is that children grow up in loving homes and never have to personally experience the depths of sin. It is God's will that loving Christian homes reproduce themselves in the next generation and the next etc. until the world is filled with healthy, loving homes serving God. Satan, of course, does not like this plan and seeks to destroy it by twisting sexual desires from a good thing to an evil thing.
The proper experience of sex within marriage is part of God's plan in preparing us for eternity. No matter how long we live on this planet it is only a blink of the eye compared with living forever in the presence of God after death. Everything in this life is designed to train and prepare us for the greatness of our life after death. Properly controlling and using our sexual desires is one aspect of our training. It develops character [or else reveals character defects which need to be dealt with]. Our actions and reactions around this intense emotional and physical experience bring many things to the surface.
Finally, limiting sex to marriage increases the partner's enjoyment of each other. They can give themselves completely, lovingly, and with total confidence to the other person. In a healthy relationship there is no fear of abandonment or rejection. Unfortunately, even many marriages do not reach this level, but we should all be working toward it. A husband who selfishly requires sex from his wife leaves her feeling abused and taken advantage of. Sexual intimacy is only one part of a total caring relationship.
Side note: The Bible never instructs us to fight sexual temptation. The command is: FLEE. Run. Do not try to argue with it. Don't try to reason with it. Don't think you are strong. RUN. Do everything you can to avoid it. Look for God's way of escape. It will always be there.
Also, read the article: Better microbes: nature abhors promiscuity.
Study Guide: Why Did God Limit Sex To Marriage?- 8 pages