Leviticus - Chapter 18

This chapter contains God's laws about personal relationships.  The seventh of the ten commandments said "you shall not commit adultery".  In this chapter there are a lot more details about how God's people were to live.

Jesus addressed this subject in his Sermon on the Mount - in Matthew chapter 5.  There, rather than relaxing the rules, he made it clear that people should take personal purity seriously:

“You have heard the commandment that says, ‘You must not commit adultery.’ But I say, anyone who even looks at a woman with lust has already committed adultery with her in his heart. So if your eye—even your good eye—causes you to lust, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell."