Sex is intrinsically tied to marriage in the Old and New Testaments. Sex outside of or before marriage is called fornication, and it is sin.

The Bible says, “Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire” (Jude 1:7).

“For this is the will of God, [even] your sanctification, that ye should abstain from fornication” (1 Thessalonians 4:3).

“Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry: For which things’ sake the wrath of God cometh on the children of disobedience” (Colossians 3:5, 6).