Naturally, like most children indignant at the withholding of something pleasurable, we want to know why God puts such restrictions on sexual expression. Probe Ministries has an excellent article entitled Why Wait Till Marriage that explores several common arguments in favor of premarital sex by showing the fallacies in those arguments.
To Probe's article, I would add that the marriage covenant reflects the mystery of Christ's relationship with His Church:
22 Wives, submit to your own husbands, as to the Lord. 23 For the husband is the head of the wife even as Christ is the head of the church, his body, and is himself its Savior. 24 Now as the church submits to Christ, so also wives should submit in everything to their husbands.
25 Husbands, love your wives, as Christ loved the church and gave himself up for her, 26 that he might sanctify her, having cleansed her by the washing of water with the word, 27 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. 28 In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself. 29 For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church, 30 because we are members of his body. 31 “Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.” 32 This mystery is profound, and I am saying that it refers to Christ and the church. 33 However, let each one of you love his wife as himself, and let the wife see that she respects her husband. ~~Ephesians 5:22-33 (ESV)
I don't pretend to understand this analogy completely (even Paul calls it a profound mystery), but I certainly get that Christ wants us to belong exclusively to Him. Several passages of Scripture, too numerous to list here, make a correlation between dalliances with false worship and adultery. And, since one can't be 100% sure that marriage will take place until he or she actually makes public vows and signs a legal contract, sexual activity indeed constitutes adultery against that other person's eventual spouse, as well as one's own future spouse. So sex within heterosexual marriage mirrors the intimacy and exclusivity of Christ's relationship with His Church.
Yet the Huffington Post reports that 80% of young unmarried evangelicals engage in sexual activity, seeing little or no contradiction between their behavior and Scripture's teaching. Though many of them may ask, "Why wait?'', I suspect few of them honestly want the answers.