There is a clause of darkness when we reflect on this topic. The question is if those who are intending to marry can sleep together. Sleeping together before marriage has a clause of sin hidden in it. Altogether, sleeping together doesn't mean having intercourse. It might only mean spending the night together or having to sleep on the same bed over night.
Situations can warrant that too people who are courting could have to spend a night together. They can sleep together and wake up without uncovering the nakedness of each other. This is not a sin. But the clause in this action is that nobody who hears that will believe that they didn't have intercourse together over the night. So, it is wise that intending couples manage their closeness wisely so that they won't be blamed.
On the other hand, it is sinful for people who are just to marry if they start having sex together. We are christians. In the doctrines of our religion, we have some discipline which we have to observe. These practices are not convenient but we subject the passion in our flesh and obey God. It is obedience to the word of God that will give us the kingdom of God. So, having sexual intercourse when you have not married yet is a sin. That is fornication.
