One of the many reasons...

 I came across this today, and this is good.  The last paragraph is something I have never thought of before.  I just have to share it with you.

Why NO to sex outside marriage….and though I have personally found there are many, many reasons to say wait until marriage, this last paragraph was one I hadn't considered:

Ephesians 5:24-32 pointedly describes the sexual relationship within marriage as an image of the spiritual relationship between Christ and the church. As you read the passage, note carefully the significance of the last sentence (verse 32) within its context:

Now as the church submits to Christ, so also wives should submit in everything to their husbands. Husbands, love your wives, as Christ loved the church and gave himself up for her, that he might sanctify her, having cleansed her by the washing of water with the word, so that he might present the church to himself in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish. In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself. For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church, because we are members of his body. "Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh." This mystery is profound, and I am saying that it refers to Christ and the church.

From Paul's comments in Ephesians we can see that when a man and a woman come together sexually, in some mysterious way they become one in their flesh (see also 1 Corinthians 6:16). Something profound occurs through sexual intercourse. The marriage union is not simply a legal union or a social union, a financial union or a familial union, but rather a union of bodies, a sharing of physical life. Through sex, two people are joined together in the deepest and most wonderful way—so much so that they are said to become one. This is why sexual intercourse is rightly said to "consummate" a marriage.

Marriage is more than sex, but it's not less than sex. In fact, in the ancient biblical world, sexual union was the primary means by which a man and woman married each other (see, for example, the marriage of Isaac and Rebekah in Genesis 24:67). Unlike today, religious clergy of the ancient world did not create a marriage through a formal pronouncement; rather the act of sex itself created the marriage. Thus, a healthy marriage relationship is the living out of the union that is established through sexual intercourse. (This is why a sexual relationship that occurs outside the context of a marriage relationship is so emotionally destructive. The act of sex, which is meant to initiate and sustain a permanent union of marriage, is broken apart and divorced from its very purpose.)

Adapted from Sex, Dating, and Relationships by Gerald Hiestand and Jay Thomas, © 2012, pp. 17-26.