God intended that sex be enjoyed between a husband and wife in marriage, so couples who have sex outside of marriage would be considered a sin. Even if you live together and plan to get married, having sex before marriage is still a sin and is not pleasing to God. The Bible is clear that marriage should be honored by all, and that the marriage bed should remain undefiled, as God will judge those who are sexually immoral and adulterous. When asked what the Bible says about sex, most people will have this answer.
But when asked to provide exactly where this rule appears in the Bible, many Christians' answer is much less certain. When a person sleeps in the same bed as a person who is not their spouse, they are also open to many temptations. All quotes from the Scriptures, unless otherwise indicated, are taken from The Holy Bible, the standard version in English. Simply having sex or living together does not fulfill the functions of the biblical model of marriage.
If you're looking for more information on what the Bible says about sex before marriage, you can visit the Bible online to search for words.