It is not advisable for unmarried people to sleep together. A boy friend and a girl friend should not sleep together because it is wrong. Even though those people might have the intention of getting married, they should avoid getting too close to the point of sleeping together and having sexual acts. That is a sin. People who are not yet married can get along but they must avoid sleeping together. They must keep themselves from performing sexual acts together before marriage.
Sometimes, situations can warrant too people who are not legally married to stay in a place alone. They might even sleep together. But they only sleep together with performing any sexual acts then they are not doing anything wrong. It is a sinful act when people who are not married have sex together. But if they do not have sex together, they are not committing sin. But to avoid temptations, it is advisable that intending couples or unmarried people avoid getting too close to the point of spending the night together alone.