Is it a sin to imagine or to daydream or fantasizing about taking a nap with your girlfriend or imagining that she is your wife all ready and thinking about what it would be like to sleep with her or wake up next to her. I'm not talking about sex or any kind of sexual act, just the thought of what marriage might be like and what it would be like to sleep with your wife (literal sleep). Is this still lust or sexual desire, even if you are not picturing sex? Of course I would never spend the night at her house or let her stay with me at my house if we are not married. Is this sinful?
There is nothing wrong with daydreaming about things that are not sinful. A husband is expected to sleep with his wife. But there can be a danger in such thought leading to more intimate thoughts that are not appropriate when you are not married.
For example, the heroine in Song of Solomon began to daydream of cuddling closely with her fiance. "His left hand is under my head, and his right hand embraces me" (Song of Solomon 2:6). But she breaks out of her role to warn the women in the audience that such daydreams were too soon. "I charge you, O daughters of Jerusalem, by the gazelles or by the does of the field, do not stir up nor awaken love until it pleases" (Song of Solomon 2:7). It is a form of rushing the relationship.
Since the marriage has not taken place, you can't guarantee that you will one day wake up lying next to this particular woman. You can't rush things because of your desires. You have to let the relationship develop at its own pace. That you have such a desire is fine, but you can't dwell on it or consider it a done deal.