Oct 10, - What is written about sex before marriage in the Bible comes predominantly from the book of 1 Corinthians, written by Paul. 1 Corinthians says “Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body. Jun 15, - Does the Bible teach that sex before marriage is a sin? A lot of people aren't sure. This is because we live in a world of tweets and quick.