It’s good to look at the following 3 passages together:
Genesis 2: 24Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall BECOME ONE FLESH.
Matthew 19:5 ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall BECOME ONE FLESH’? 6 So they are no longer two but one flesh. What therefore God has joined together, let not man separate.
1 Corinthians 6 16 Or do you not know that he who is joined to a prostitute BECOMES ONE BODY WITH HER? For, as it is written,“The two will BECOME ONE FLESH.
It certainly seems that the sexual act creates the one flesh - the permanent relationship. The only question there is, is this done in the correct, God-honouring way? And can an incorrect relationship be subsequently corrected?
A few years ago, I was organist at a wedding for a woman who had been living with a man for over 20 years. She had cancer and during that time had become closely involved with someone from my local church who ran one of the women’s Bible studies. She became a Christian and decided that she wanted to make things right with God in terms of her relationship, hence the wedding and the vows before God, also as a witness to her mostly non-Christian friends and colleagues. I played the organ at her memorial service about 3 months later. It would seem to me that they were one flesh for life right from the start of the relationship, but then turned it into a right relationship for those last few months.
Regarding the question of rape also mentioned in one of the posts, the punishment for rape was death thus freeing the woman concerned from the “until death” relationship.
The one flesh enables sexual union. Animals and fornicators are capable of are capable of raw sex.