QUESTION: Where in the Bible does it say that it is a sin to have sex before marriage?
ANSWER: Sex is an important part of life. It should not come as any surprise that sex attracts a lot of interest and raises many questions. Your question suggest that there might be a verse somewhere in Scripture that answers the question. Scripture, however, is not a list of rules and regulations, but a story about the love of a Creator God who has the best in mind for his creation.
The common word for sex before marriage in the NT is porneia. It occurs 26 times in the NT and is translated by different English words in different translations (fornication in the KJV and sexual immorality in the NIV). It’s usual meaning is to participate in sexual intercourse outside of marriage.
In 1 Corinthians 6.18, Paul exhorts the members of the Corinthian church to flee from sexual immorality (porneia) after telling them that having sex with a prostitute (i.e., having sex with one with whom a person is not married) is not acceptable behavior for a believer.
There is probably a lot of preachy stuff out there about the subject of premarital sex which is a great turn off to those who are struggling with their sexual appetites. It may be well to remember that those who call themselves believers in Christ are called to live by a different standard of life than those who are not believers. When faced with a situation of having sex with a partner to whom you are not married, you might ask yourself the question. If Jesus were you in that very situation how would he act.