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Sex before marriage is a sin.
1 Corinthians 6:9 "I wrote to you in an epistle, not to keep
company with fornicators."
1 Corinthians 6:18 "Fly fornication. Every sin that a man doth,
is without the body; but he that committeth fornication, sinneth
against his own body."
Extramarital sex is a sin.
Exodus 20:14 "Thou shalt not commit adultery."
Deuteronomy 5:18 "Neither shalt thou commit adultery."