Is it wrong for a couple to live together before marriage?

The answer to this question depends somewhat on what is meant by
"living together." If it means having sexual relations, it is
definitely wrong. Premarital sex is repeatedly condemned in Scripture,
along with all other forms of sexual immorality (Acts 15:20; Romans
1:29; 1 Corinthians 5:1; 6:13, 18; 7:2; 10:8; 2 Corinthians 12:21;
Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3;
Jude 7). The Bible promotes complete abstinence outside of (and
before) marriage. Sex before marriage is just as wrong as adultery and
other forms of sexual immorality, because they all involve having sex
with someone you are not married to.

If "living together" means living in the same house, that is perhaps a
different issue. Ultimately, there is nothing wrong with a man and a
woman living in the same house—if there is nothing immoral taking
place. However, the problem arises in that there is still the
appearance of immorality (1 Thessalonians 5:22; Ephesians 5:3), and it
could be a tremendous temptation for immorality. The Bible tells us to
flee immorality, not expose ourselves to constant temptations to
immorality (1 Corinthians 6:18). Then there is the problem of
appearances. A couple who is living together is assumed to be sleeping
together—that is just the nature of things. Even though living in the
same house is not sinful in and of itself, the appearance of sin is
there. The Bible tells us to avoid the appearance of evil (1
Thessalonians 5:22; Ephesians 5:3), to flee from immorality, and not
to cause anyone to stumble or be offended. As a result, it is not
honoring to God for a man and a woman to live together outside of
marriage.