Is sex before marriage a sin?

The Bible says fornication is a sin “against his own body”:

1 Corinthians 6:18 King James Version (KJV)
18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.

The book of 2 Corinthians writes about those who “have sinned already, and have not repented of the uncleanness and fornication and lasciviousness which they have committed”:

2 Corinthians 12:21 King James Version (KJV)
21 And lest, when I come again, my God will humble me among you, and that I shall bewail many which have sinned already, and have not repented of the uncleanness and fornication and lasciviousness which they have committed.