The answer to this question depends to some extent on what is meant by
“living together” or cohabitation. If it doesn’t mean having sex, then it’s definitely wrong. Premarital sex, along with other forms of sexual immorality, is repeatedly condemned in Scripture (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 absolutely encourages abstinence outside of (and before) marriage. Sex before marriage is just as wrong as adultery and other forms of sexual immorality, because it all involves someone you are not married to.
If “living together” or companionship means living in one house, then perhaps that is a different matter. Finally, there is nothing wrong with a man and a woman living in the same house – if no immorality occurs between them. However, there can be a real problem because it also shows immorality (1 Thessalonians 5:22; Ephesians 5:3), and this method can lead to the very temptation of immorality. The Bible tells us to flee from immorality, not to expose ourselves to temptations of immorality (1 Corinthians 6:18). Again here is the problem of appearance. A couple that cohabits is presumed to sleep together – this is by nature. While they do not live in the same house and are full of sin in themselves, there is an appearance or manifestation of sin. The Bible tells us to live from the appearance of evil (1 Thessalonians 5:22; Ephesians 5:3), to flee from immorality, And it says not to be a cause of stumbling or stumbling to others. Consequently, it is not God-honoring for a man and a woman to live together without marriage.