This question could be much more easily answered if the Bible made a clear proclamation such as “living together before marriage or outside of marriage is living in sin.” Since the Bible makes no such definitive statement, many (including some who profess to be Christians) claim that living together outside of marriage is not living in sin. Perhaps the reason the Bible does not make a clear statement is that in Bible times, unmarried people living as husband and wife was relatively rare, especially among the Jews and Christians. For the purposes of this article, when we refer to living together, we are referring to living together in the sense of living as husband and wife, including sexual relations, without being married. We are not referring to a man and woman living in the same house without sexual relations. That issue is covered in another article.
While the Bible does not make an explicit statement about living in sin, that is not to say the Bible is completely silent on this issue. Rather, we have to put several Scriptures together and glean from them the principle that any sexuality outside of the marriage of one man and one woman is definitely sin. There are numerous Scriptures that declare God’s prohibition of sexual immorality (Acts 15:20; 1 Corinthians 5:1; 6:13, 18; 10:8; 2 Corinthians 12:21; Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3; Jude 7). The Greek word translated “sexual immorality” or “fornication” in these verses is porneia (from which we get the English word “pornography”), and it means literally “unlawful lust.” Since the only form of lawful sexuality is the marriage of one man and one woman (Genesis 2:24; Matthew 19:5), then anything outside of marriage, whether it is adultery, premarital sex, homosexuality, or anything else, is unlawful, in other words, sin. Living together before marriage definitely falls into the category of fornication—sexual sin.
Hebrews 13:4 describes the honorable state of marriage: “Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral.” This verse draws a clear distinction between that which is pure and honorable—marriage—and that which is sexually immoral—anything outside of marriage. As living together outside of marriage falls into this category, it is definitely sin. Anyone living together outside of lawful marriage invites the displeasure and judgment of God.
 Got Questions Ministries. (2010). Got Questions? Bible Questions Answered. Bellingham, WA: Logos Bible Software.