I couldn’t even find a passage saying that premarital sex is not allowed.
In the Mosiac Law, premarital sex is mentioned in Deuteronomy 22:28-29:
If a man meets a virgin who is not betrothed, and seizes her and lies with her, and they are found, then the man who lay with her shall give to the father of the young woman fifty shekels of silver, and she shall be his wife, because he has violated her; he may not put her away all his days.
In the New Testament the word
fornication is used for premarital sex.
For out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander. These are what defile a man; (Matthew 15:19-20)
For from within, out of the heart of man, come evil thoughts, fornication, theft, murder, adultery, coveting, wickedness, deceit, licentiousness, envy, slander, pride, foolishness. All these evil things come from within, and they defile a man." (Mark 7:20-22)
Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God. (Galatians 5:19-21)
But fornication and all impurity or covetousness must not even be named among you, as is fitting among saints. Let there be no filthiness, nor silly talk, nor levity, which are not fitting; but instead let there be thanksgiving. Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God. Let no one deceive you with empty words, for it is because of these things that the wrath of God comes upon the sons of disobedience. (Ephesians 5:3-6)
Put to death therefore what is earthly in you: fornication, impurity, passion, evil desire, and covetousness, which is idolatry. On account of these the wrath of God is coming.
(Colossians 3:5-6)