Sex outside of marriage is classed as fornication.
Galatians 5:19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
1 Corinthians 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
Marriage is the means whereby we avoid fornication.
1 Corinthians 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
Those who believe the gospel and are baptised into Jesus Christ must avoid sex outside of marriage.
Ephesians 5:3 But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints;
Colossians 3:5 Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry:
1 Thessalonians 4:3 For this is the will of God, even your sanctification, that ye should abstain from fornication:
God’s will is that both male and female should remain virgins until they are married.
The model is Christ and his bride (the Church).
2 Corinthians 11:2 For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ.
We owe it to our future spouse to remain celibate before marriage.
I hope you find this helpful.