A straightforward scripture on the subject of sex outside of marriage is in Hebrews 13:4, which states, "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge."
The implication is that while the marriage bed (sexual union between a husband and wife) is undefiled, the act of sex between two unmarried persons is a defilement. The scripture also warns that those who commit fornication and adultery (premarital and extramarital sex) will be judged by God. In simplest terms, the only person God permits us to have sex with is our spouse—the person we have committed ourselves to before God for life.
Like all of God's commands, there is nothing arbitrary or archaic about the command to save sex for marriage. Countless studies and experiences, some of which are examined in the material below, enforce the idea that God designed sex for a husband and wife only. Outside of the marriage relationship, sex categorically weakens relationships—but inside marriage, sex is designed by God to strengthen and renew the bond between a husband and wife.
For more insight on this topic, please read our booklet "Marriage and Family: The Missing Dimension".