It is understandable that a couple would want to lie down together. After all, who would not want to wake up next to their loved one? However, this kind of intimacy belongs only in marriage. To lie down with a woman in bed is marital. When we use the expression, “They slept together,” we usually are not thinking about sleep at all. But this phrase is used because the marriage bed is reserved for the marital act.
In Hebrews 13:4 we are told to keep the marriage bed undefiled. It is to be sacred, and this means it is to be set apart for holy use. The holy use that God has in mind is marital union. In your heart you know that this belongs in marriage, because if you knew that sleeping in the same bed with her was pleasing to God, you would not have asked this question.
If you want to save sex for marriage, sleeping in the same bed is not the best way to guard that commitment. Granted, you may not be having sex, but as Proverbs 16:18 says, pride goes before a fall. There is wisdom in avoiding occasions of sin and not trusting ourselves too much.
Promise each other that the next time the two of you share a bed, it will be as husband and wife. There is a time and a season for everything under the sun, and as difficult as it may be, purity calls you to make this sacrifice. After all, if the Lord calls you to marry her, you will have the rest of your life to fall asleep looking at her.