The very nature of a male-female relationship is that of companionship, friendship and intimacy. The marriage relationship is supposed to last a lifetime, and to make that possible, people get to know one another in the period leading up to the marriage. They are not committed to the one another legally, morally or otherwise. They do not live together and do not have mutual responsibilities. They are free to spend time together and learn what makes each other tick, free to question and discuss and debate. They are able to make clear-headed choices about spending their lives together. When the growing intimacy between them leads to the altar, they know what they are getting. After the wedding, they are free to seal the relationship as only a husband and wife can do.
The above process comprises the foundation for building a lifelong relationship. When sex enters the scene too soon, the foundation is weakened and frequently collapses completely. As many [researchers] have stated, sex takes over dating relationships like wildfire. It consumes everything built to that point.
One of the first casualties is communication. When a couple has found a quick way to be “intimate” by having sex, they don’t bother taking time to become mentally and emotionally and spiritually intimate. They become lazy in their attempts to grow closer, opting for what is easy over what is lasting.
The next casualty is trust. When people have revealed their character to each other by having sex before marriage, there is distrust on both sides. Each knows the other is incapable of controlling their physical desires. What will happen if they should be apart for a while? What is to keep each from seeking out other opportunities for gratification? Frequently, nothing. Whether they say it out loud or not, they know it.
One by one, the vital mutual concerns needed to built a lasting intimate relationship are consumed by sex. Sex displaces and then destroys love.
Josh McDowell and Dick Day, "Why Wait?" p.292-293