The Bible does say that premarital sex is a sin. Many times. Here is one of them:-

Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God. – 1 Corinthians 6:9-10

This mentions adultery, but it also mentions something different from adultery: sexual immorality, or what is sometimes translated into English as “fornication”. The meaning of “fornication” is clear enough, but what some people will do is argue that the original Greek word, pornea, has been misinterpreted. The word basically means “a forbidden sexual pleasure”, and some people have argued that it had a very narrow meaning and referred only to sex that was part of idol worship. Those people say that it is fine to have premarital sex, as long as you are not doing it in a pagan temple.

It is essentially impossible that Biblical authors wrote the word with that suspiciously narrow definition in mind, though. Look at how Paul, the author of 1 Corinthians, used the exact same word just a few sentences later:

But since there is so much immorality (pornea), each man should have his own wife, and each woman her own husband. – 1 Corinthians 7:2

Paul is an extremely logical person, and his practical solution to the problem of pornea was to get married. Why was that the solution? Because within marriage you can have sex. He actually spends the rest of that paragraph encouraging married people to have sex together more often, and specifically tells the unmarried that “it is better to marry than to burn with passion” if they find they “cannot control themselves” (verse 9).

If he was talking specifically about idols, marriage would not be the solution; you can be married and worship idols. He is clearly and very frankly talking about sexual release within marriage as a solution to the problem. And if Paul’s logical solution to the problem is sexual release, why doesn’t he offer premarital sex or masturbation as options? Wouldn’t those be the most practical solutions to the problem of pornea?

The reason he doesn’t list premarital sex as a solution is because premarital sex is the problem. Premarital sex is pornea, and it is pornea whether it happens in a temple or not. That is why his solution to the problem, for those who could not control it, was marriage.