It's a question people in love, and sometimes people who are just "in like," or lonely, ask. There's a whole range of answers.
But aren't earnest questions about sex often questions about values? And when values are important to you, intellectual reasoning alone usually won't result in satisfying answers. It doesn't allow for the one factor that is more valuable than anything else - your relation to God.
From the moment you acknowledge that God is real - not just something other people have made up to regulate human behavior - God becomes a factor, and more and more the leading factor, in every significant decision you make.
So the natural basis from which to begin seeking an answer to any important question is "What is God?"
Why not take a moment to consider your honest answer to that question. What's the most tangible idea of God to you? Now, keeping this concept in mind, consider how you fit in with it. I'm going to give an example here, but you can plug your own thought of God into this reasoning: God is universal, all-loving, divine intelligence, filling all space. How do you fit in? This intelligence is your actual substance and individuality. You are primarily and eternally a distinct idea of God. Your life purpose is very precious to God because you exist to express Him.
A friend of mine discovered a connection between his relation with God and his attitude about sex. He had a couple of romances where the physical attraction was very strong. These relationships also had a lot of emotional ups and downs that consumed a great deal of his attention. During this time, however, he was also growing in the desire to be faithful to God. He knew that this involved commitment to habits of self-control and spiritual thinking and living. Just doing whatever he felt like, whatever his body said he wanted or needed, wasn't helping him form those essential habits.
A beatitude in Christ's Sermon on the Mount spoke to him very directly: "Blessed are the pure in heart: for they shall see God" (Matt. 5:8). A few times in his life he had "seen God," that is, he'd felt a closeness to Him that was wonderful and powerful.
In his clearest moments, my friend knew that what he wanted more than anything else was to feel that light of God's presence more regularly. He decided that one step toward the purity of heart that sees God was to abide by the Christian standard of no sex outside of marriage. It wasn't always easy, but he never doubted that he had chosen the right course.
Sometimes people feel that such standards are arbitrary and even hypocritical. Certainly, marriage isn't a magic step that somehow makes sex pure. But marriage does signify a lasting commitment to love another person in ways that go beyond sexual gratification. This responsibility lends a protecting trust to intimacy. And even within marriage, self-control continues to be necessary to spiritual progress.
Writing on marriage, Mary Baker Eddy, who discovered Christian Science, says: "Both sexes should be loving, pure, tender, and strong. The attraction between native qualities will be perpetual only as it is pure and true, bringing sweet seasons of renewal like the returning spring" ("Science and Health with Key to the Scriptures," Pg. 57).
Purity encompasses much more than sexual behavior, of course. It might be defined as devotion to God, to what is spiritual and permanent. Recognizing our natural, God-derived purity affects our behavior in many ways. It causes us to be honest, to care about justice, to protect the environment. It also results in thoughtful and willing obedience to standards of conduct that have come to humanity through inspiration and revelation. If we compare the goal of seeing God to achieving excellence in a sport, isn't maintaining purity like the disciplined practice required of champions?
This way of living is actually normal. Sometimes we just need to be reminded of how much God means to us, and how much we mean to Him.
Why wait? It's hard to imagine a more important reason than to have the pureness of heart that sees God and blesses our lives with His goodness.