S** outside of marriage causes damage in at least two areas: (1) physical consequences, and (2) relational consequences.
 				The physical consequences are becoming increasingly obvious and increasingly dangerous in today's society. AIDS and other Sexually Transmitted Diseases are frightening realities. “Safe s**” is more accurately described as “reduced risk s**.” The only truly safe *** is abstinence. There is also a very real risk that children could be born — and possibly grow up without two parents. Your actions affect your life, your partner's life, and the lives of your family. They can result in handicapping an innocent baby's life as well. Worst of all the willfull destruction of 
human life often results from pre-marital s**.)
 				The relational consequences are just as real, though they may be more difficult to grasp. First, sin always damages a person's relationship with his God. 
Psalm 66:18 says, "If I had cherished sin in my heart, the Lord would not have listened." Intentional disobedience of God's command to not commit 
adultery dishonors and displeases God. Conversely, God is pleased when His children choose obedience and self-control instead of the immediacy of pleasure.
 				Second, relational damage happens between a Christian and those who are watching his life. The sin of 
adultery (i.e., televangelist scandals) causes a person's friends and even “outsiders” to view the adulterer as less committed to obedience, and more prone to hypocrisy. But a Christian who saves himself or herself in obedience to God wins the respect of those who see his or her life.
 				*** outside of marriage also damages the relationship between the persons involved. Trust is the main issue here. If two people do not cherish s** enough to wait for a marriage commitment, how can they trust one another for fidelity? Conversely, a man and woman build trust and respect for one another when they both survive the struggles of self-control — each will have the confidence that the other respects them, and cherishes their intimacy.
 				Similarly, if a person has not carried sexual purity into marriage, his or her marriage relationship is affected by the past. If a man or woman has previously had s** with someone else, their marital intimacy has already been affected. One or both spouses will have to deal with real or perceived comparisons with “former lovers” and feeling that intimacy was not important enough for the other person to wait for it. But if both have waited for their wedding night, the intimacy has already begun with a solid foundation.
Why should i save S** for marriage?
Why should I save *** for marriage? - ChristianAnswers.Net