No, what Scripture says (how did you miss it) is not that two persons of opposite sex automatically become married just because they engaged in sexual relations (which would make many of us polygamous), but instead even before the Law fornication was not marriage,[1] and under the Law such fornicators were required to become married,[2] and Christ taught that the women at the well was not married even though she was living in sin with a man:[3]
[1] And Dinah the daughter of Leah, which she bare unto Jacob, went out to see the daughters of the land. And when Shechem the son of Hamor the Hivite, prince of the country, saw her, he took her, and lay with her, and defiled her...And the sons of Jacob came out of the field when they heard it: and the men were grieved, and they were very wroth, because he had wrought folly in Israel in lying with Jacob’s daughter; which thing ought not to be done. And Hamor communed with them, saying, The soul of my son Shechem longeth for your daughter: I pray you give her him to wife... (Genesis 34:1-2,7,8) And they slew Hamor and Shechem his son with the edge of the sword, and took Dinah out of Shechem’s house, and went out...And they said, Should he deal with our sister as with an harlot?(Genesis 34:26,31)
[2] And if a man entice a maid that is not betrothed, and lie with her, he shall surely endow her to be his wife. (Exodus 22:16)
If a man find a damsel that is a virgin, which is not betrothed, and lay hold on her, and lie with her, and they be found; Then the man that lay with her shall give unto the damsel’s father fifty shekels of silver, and she shall be his wife; because he hath humbled her, he may not put her away all his days. (Deuteronomy 22:28-29)
[3]Jesus saith unto her, Go, call thy husband, and come hither. The woman answered and said, I have no husband. Jesus said unto her, Thou hast well said, I have no husband: For thou hast had five husbands; and he whom thou now hast is not thy husband: in that saidst thou truly. (John 4:16-18)
" Consider that all the ceremony (not celebration and feasting, that’s separate from things like exchanging vows and is much earlier) was added later, and even as late as the Patriarch period we expressly see no ceremony at all with, for example Isaac and Rebekah (in that case not even a celebration is spoken of)."
The celebration and feasting was because there was a social contract taking place, with the community itself being involved. And which is what "yes" to the question "Wilt thou go with this man?" signified, celebration or not.
And they called Rebekah, and said unto her, Wilt thou go with this man? And she said, I will go. And they sent away Rebekah their sister, and her nurse, and Abraham’s servant, and his men. And they blessed Rebekah, and said unto her, Thou art our sister, be thou the mother of thousands of millions, and let thy seed possess the gate of those which hate them. (Genesis 24:58-60)
Thus your assertions do not stand in the light of comparing Scripture with Scripture, which shows that while "he which is joined to an harlot is one body" (1 Corinthians 6:16) - which make that women a polygamist - yet that is only one aspect of becoming married.
Did you not note the idea as the alternative of adultery?
Did you not note that becoming one flesh applies when those involved are not married? Or at least (because of OT polygamy) when she is not married and is eligible to marry the man?
Or did you note that I did specify the era of the patriarchs to show that ceremonies as a necessary component came later in Israel?
Dinah’s fate is not explicitly settled by Scripture BUT some extra-Biblical Jewish claims, for what that is worth, associate the “Canaanite woman” of 46:10 who was Shaul’s mother with her, making Shaul the son of Shechem raised in Simeon’s house and counted among his sons. If so then she was considered his, Shechem’s, widow to be described that way.
Simeon and Levi you may recall took the lead in vengeance for her treatment, not Reuben who was the oldest brother to all three, and Simeon was older than Levi ... so whatever the truth may be her ending up under Simeon’s protection in his house at least makes sense.
Naturally, the Law addressed and formalized issues that may not have been addressed under the patriarchs ... after all, Simeon and Levi applied a very different standard of justice than Exodus 22 or Deuteronomy 22.
As for what Jesus said, that she had had 5 husbands ... and they were really her husbands ... would mean that she was free to marry numbers 2 through 5 but NOT the sixth. That could be satisfied if she was widowed by 1 through 4, but 5 was still alive ... thus she was an adulterer with the current guy ... and any potential in betweeners betwixt him and her still living husband..
If she were divorced from 1 to 4, or indeed from 5, she would run afoul of the teaching of Luke 16:18, also from Jesus. So she might have been divorced from #5 but that didn’t make her eligible to marry again.
I think my assertion hold up quite well.