One flesh is caused by sexual intercourse, and nothing more. But one flesh itself is the state of being metaphorically and spiritually one body with another. The wife's body does not belong to her, but to her husband, and vice versa, because they are one flesh and their bodies are the same. (1 Cor 7). Similarly, we are one in spirit with Christ, and our bodies do not belong to us, but to Him, as His body belongs to us, as He gave it to us on the cross. (1 Cor 6:17) Paul says this is a mystery, but in the Greek, mystery does not mean "something that is unknowable." HELPS Dictionary says this about mysterion:
the Bible, a "mystery" (3466 /mystḗrion) is not something unknowable. Rather, it is what can only be known through revelation, i.e. because God reveals it.[some]
Consequently, ALL mysteries are only mysteries after they have been revealed, for we don't know about them until they are revealed to us!
"Marriage is a mystery." Christ, in His divinity, revealed that His relationship to us is analogous to a marriage, and that is the full extent of the "mystery" of marriage.One flesh ties into all of that because we are one spirit with Christ. We enter into a one spirit relationship with a singular act of faith, and similarly, for our bodily unions we enter into a one flesh relationship with a singular act of physical union. That is, sex.
1 Cor 6:16 Or do you not know that anyone who is united with a prostitute is one body with her? For it is said, “ The two will become one flesh .”
The grammar Paul uses here suggests that he is equating one body to one flesh, as the transitional word gar is used, which denotes causation. This gives us the assurance that sex causes two people to become one flesh.
Matt 19:3-6 Then some Pharisees came to him in order to test him. They asked, “Is it lawful to divorce a wife for any cause?” He answered, “Have you not read that from the beginning the Creator made them male and female , and said, ‘ For this reason a man will leave his father and mother and will be united with his wife, and the two will become one flesh ’? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”
Jesus says that it is God who puts together two people into one flesh. According to Paul, this means that God puts together those who are one flesh whenever they have sex. And then Jesus states that what God has put together, let no man separate.
The ramifications of this are not hard to see. If sex makes you one flesh, and those who are one flesh are not to separate, then that means they must cleave to each other. Sounds a bit like a marriage, does it not? And considering that Jesus was speaking on divorce, it seems likely that He was insinuating that you become married, and thus able to divorce and commit adultery in God's eyes upon having sex. This certainly would explain the plethora of verses on adultery and adulterous people in the Old Testament and the great lack of passages of premarital sex.
The story that speaks the most evidence into this definition of one flesh equating to sex equating to marriage is the story of Jacob, Rachel, and Leah in Gen. 29.
Jacob and Rachel were betrothed, which in Hebrew culture meant they were husband and wife by covenant (v. 21), but they did not have sex. According to Hebrew custom, Jacob paid a dowry to Laban, the father, for Rachel (v. 18). Then they held a wedding feast for Jacob and Rachel with many people there, presumably also within Hebrew customs (v. 22). According to our standards today, Jacob and Rachel were at this point more than married enough to be considered united and separated from Laban. They were married under a covenant, and they had a wedding. Yet when Jacob has sex with Leah, we see that he has to pay another 7 year dowry for Rachel, again! Apparently those dowries went to whoever you ended up having sex with, not the person you made a covenant with. The covenant meant basically nothing in terms of the right to "own" Rachel, it would seem.
From the other angle, let us consider Leah's marriage to Jacob. Leah and Jacob were not betrothed, or Jacob would not have been angry upon having sex with her (v. 25). The wedding was not for Jacob and Leah, or Jacob would have known about the deception. Yet when Leah and Jacob have sex, apparently the dowry transfers to her, and Jacob apparently has to agree to keep her as his wife in order to obtain Rachel. Sounds an awful lot like sex made them married, despite the lack of covenant.
Now let's look at the story from Laban's perspective. Laban knew all along that he was going to deceive Jacob into having sex with Leah. But here's the thing: what purpose would deception serve if sex has no inherent commitment? Laban would essentially just lower Leah's virgin status needlessly, as the sex would constitute just premarital sex, and he would have angered his potential son-in-law needlessly, and in this day, such a thing was potentially mortal in danger.
No, the only way Laban's plan of deceit makes sense is if he has the capability of "tricking" Jacob into marrying Leah, which points directly to the idea that sex is marriage. Conversely, the only thing lacking in Jacob and Rachel's marriage was sex, and without that, Jacob could not have left Laban's household without her.
Now there are laws in Exodus 22:16-17 and Deuteronomy 22:28-29 that show us that Hebrew law dictated exactly these consequences for this situation, but we cannot simply say that sex is marriage here because the law said so. No, we must say the law said so because sex is marriage, because this story happens hundreds of years before those laws were ever given!
The ramifications are far reaching, and there is far more discussion to be had about the idea that sex is one flesh is marriage, as well as many more proofs, but I will leave it at this.
To more specifically answer your question, one flesh is begun at the point of sexual intercourse, but it is not intercourse itself. One could say that it is, along with being metaphorically the same body, the state of perpetual possibility of conception. Even with barren people, the possibility of conception is there because God is in control of conception, as we see with Abraham and Sarah. If you are opening the door to conception to God, then you are one flesh.