In 1 Corinthians 6:9-10 we read that neither sexually immoral nor adulterers will inherit the Kingdom of God. Adulterers are those who have sex outside of marriage. Sexually immoral are those who get involved in sex before marriage. Paul says that one who commits sexual immorality or adultery becomes “one flesh” with the person with whom he had sexual intercourse (1 Cor. 6:16). According to the Bible, the term “one flesh” refers to family or marriage (Genesis 2:24) “Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.” When we read about rape in Deuteronomy 22:28-29, God commands the rapist to marry the unmarried girl he dishonored. We did not see in the Old Testament that women who had previously committed sexual immorality could start a family in the future. By definition this was not possible, because they were going to be stoned to death for this iniquity. Does this mean, then, that physical attachment to a woman is seen in God’s eyes as marriage if He takes sexual immorality and adultery so seriously?
Marriage is a covenant made before God, and it is God who watches over the observance of this covenant, as we find written in Malachi:
And this is another thing you do: you cover the altar of the Lord with tears, with weeping and sighing, because He no longer gives attention to the offering or accepts it with favor from your hand. Yet you say, “For what reason?” Because the Lord has been a witness between you and the wife of your youth, against whom you have dealt treacherously, though she is your marriage companion and your wife by covenant. (Malachi 2:13-14 NASB)
The intimate relationship is part of the marriage covenant, so any physical relationship outside of marriage is seen by God as immorality / adultery.
Someone will say that Adam and Eve were not married, because they had no way to get married, because only they were on earth: they had no parents or other witnesses. However, the marriage of Adam and Eve was the most special marriage in the Universe: God Himself brought the bride to the bridegroom and witnessed their marriage!
So, marriage is a covenant between a man and a woman, and the intimate relationship is part of this covenant:
But because of sexual immoralities, each man is to have his own wife, and each woman is to have her own husband. The husband must fulfill his duty to his wife, and likewise the wife also to her husband. The wife does not have authority over her own body, but the husband does; and likewise the husband also does not have authority over his own body, but the wife does. Stop depriving one another, except by agreement for a time so that you may devote yourselves to prayer, and come together again so that Satan will not tempt you because of your lack of self-control. (1 Corinthians 7:2-5 NASB)
If physical union were seen by God as marriage, then sexual immorality (cohabitation) would not be a problem, but God says that sexual immorality as well as adultery defiles the conjugal bed:
Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for God will judge the sexually immoral and adulterers. (Hebrews 13:4 NASB)
If two young people live together before marriage, even if neither of them has any relationship with another person, their relationship qualifies as immoral and this relationship defiles their marital bed.
So, a man and a woman unite in marriage by covenant, and what makes them one is the intimate relationship, which makes the two one body. But if two people become one body through the intimate relationship, without being married, this relationship is called immoral and it defiles their marital bed and they will be judged by God.
Translated by Didina Vicliuc