Those that sleep together because of circumstances didn't do anything wrong if nothing like sexual intercause come between them but Bible condemn if man and woman sleep together do sexual act, the Bible condemn it
Matthew 1:18-20 Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.
Then Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily.
But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.
Joseph and Mary were engaged but they didn't have sexual act and that's why Joseph want to deny Mary when she had Jesus Christ pregnant
Having a fixed relationship and mutual understanding that later you would get married later is the best way of dating, not with sexual act
Sexual relationship is like a play in our society these days. Sex without marriage is called fornication according to Bible
1 Corinthians 7:1-2 Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman.
Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.