Sex before marriage is a big deal in the bible. It is called fornication. Fornication is a form of sexually immorality.
The bible states in 1 Corinthians 6:18 “Flee from sexual immorality!* Every other sin that a man may commit is outside his body, but whoever practices sexual immorality is sinning against his own body.”
For those who don’t exactly know what sexual immorality is; it is a general term for all unlawful sexual intercourse. It includes adultery, prostitution, sexual relations between unmarried individuals, homosexuality, and bestiality.