Leviticus 18 contains a series of laws that have to do with sexual expression (except for 18:21, which forbids child sacrifice). We might wonder why God didn't simply say, "Have sexual relations with your spouse only," rather than offer such detail in the form: "Do not have sexual relations with … "
The answer to this query comes in Leviticus 18:3: "So do not act like the people in Egypt, where you used to live, or like the people of Canaan, where I am taking you. You must not imitate their way of life."
God is leading the Israelites out of Egypt, where various forms of sexual immorality were common, into the land of Canaan, where the people do the very things Leviticus 18 prohibits. As verse 27 explains, "All these detestable activities are practiced by the people of the land where I am taking you, and this is how the land has become defiled." God was concerned that the Israelites would easily be tempted to imitate the Canaanites in their sexual impurity, so he spelled out graphically the relationships in which sexual activity was forbidden. He knew that his people were like young children who needed specific instructions and prohibitions.
Although our context differs considerably from that of the Israelites as they journeyed from Egypt to Canaan, we face an unsettlingly similar situation. We live today in a culture that endorses sexual activity outside of marriage. And, though our society still agrees with Leviticus 18 about some things, like the wrongness of sex with a close relative, our world presents us with new challenges to our sexual holiness, like readily available pornography.
Yet, the New Testament does not offer an updated, exhaustive list of sexual "don'ts," a new rulebook for Christian sexual ethics. Rather, it calls us to pervasive holiness in every part of life, including our sexuality (see 1 Thess. 4:3-8, for example). We are not to imitate the ways of our world when it comes to sexual expression. Rather, we are to devote our whole selves to God, including our bodies. We are to live each day with the realization that our bodies are temples for the Lord (1 Cor. 6:18-20). Thus, we have the obligation and privilege of honoring God with our bodies. When we take this high calling seriously, our desire to dishonor God by dishonoring our bodies will diminish. Rather, we will seek to give him all that we are, all the time.