Christians, both married and single, must seek to be directed by the revealed truths of Scripture. The Author of Sex Pleasure, as C.S. Lewis once pointed out, is God's invention, not Satan's. He designed our bodies with the capacity of giving and receiving sexual pleasure. The God-given sexual impulse is neither vulgar nor evil, no more than the instincts of hunger and thirst that impel us to eat and drink. Sexuality is solidly built into human nature.
Marriage is an earthly echo of that divine-plane relationship.
Conversely, sexual relations outside of marriage are a perversion of the divinely established model. Adultery is used figuratively throughout Scripture to express humanity's unfaithfulness to God. Yes, God made us sexual beings. But he also gave us clear guidelines for sexual behavior. While not fashionable in today's society, Christians are clearly called to hold a high standard of personal morality.