Marriage is a union between two opposite sex who are of marriageable age. These two have agreed to be husband and wife. It is only people who are husbands and wives that can sleep together. Sleeping together outside marriage is a sin.
We are christians and we should not indulge in sin. We should not practice sin. God hates sin. He wants us to be holy. He wants us to obey His commandments. The commandments of God says “Do not commit fornicate and adultery. We should not commit fornication. We should not commit adultery.
People who are not married should not sleep together. Those who do that are going against the will of God. It is a sin to sleep with the person whom you are not married to. Those who sin against God will have their place in hell fire
It is a good thing if we live in obedience to God's Word. Let us run away from the sin of fornication. Those who are married should remain faithful to their spouses. Our body is the temple of the Holy Spirit. It is not for fornication. We must live holy and run away from every form of sin including fornication.