If the man and woman are brother and sister and are living under the same roof, it would be permissible, provided there is nothing illicit in their relationship. If the situation is that a man and woman who are not married to each other and are living together and having sexual relations, that would be sinful. If the unmarried man and woman are living together, are not related, and are not having sexual relations, then other factors come into play. First of all, the Bible says that we are to avoid even the appearance of evil. If living together causes others to stumble then there is sufficient cause to warrant a different living arrangement.
But, basically, an unmarried man and woman living together, sleeping in the same bed, etc., is wrong and sinful.
- 1 Cor. 6:18, "Flee immorality. Every other sin that a man commits is outside the body, but the immoral man sins against his own body."
- Eph. 5:3, "But do not let immorality or any impurity or greed even be named among you, as is proper among saints."
- Col. 3:5, "Therefore consider the members of your earthly body as dead to immorality, impurity, passion, evil desire, and greed, which amounts to idolatry."
- 1 Thess. 4:2-5, "For you know what commandments we gave you by the authority of the Lord Jesus. 3 For this is the will of God, your sanctification; that is, that you abstain from sexual immorality; 4 that each of you know how to possess his own vessel in sanctification and honor, 5 not in lustful passion, like the Gentiles who do not know God."
The fact is that biblically speaking when a man and woman live together but are not married, it is a violation of God's revelation that is found in the Bible. Men and women who live together (in the context of possible marriage) are in sin. This is reserved for marriage. But the world denies God's truth and does what is right in its own eyes (Judges 21:25). The ways of the world are to seek sensual pleasures without the responsibilities that ought to go with them. In our fallen world where secularism is on the rise, to live together is a common occurrence where sexual intimacy occurs. But this is not to be the case for Christians. No Christian man and woman should live together in any way that imitates marriage. There to save that for after the wedding day.