To answer the question properly, we must first define what it means to be intimate. Since the question mentions marriage and sex, the intimacy is sexual in nature. Any type of sexual union, contact, intimacy is for the marriage only between a husband and wife. If a man and woman who are not married go to bed together naked and do not have sexual intercourse, this is still sinful. If they fondle each other without having sexual intercourse it is sinful. If they go to bed together naked and just hold each other, it is sinful. The whole point is that the nakedness, viewing the nakedness, the touching of the private areas, fondling, etc., are all reserved for the marriage bed between a husband and a wife. God clearly instructs that we remain morally pure.
- 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."
Since intimacy before marriage involves the touching and often seeing of the naked body, this is an off-limits action. Save this for the proper context of marriage.