You already know which passage but for others info : Exodus 22:16-17
This is the one exception, this is it. A betrothed woman counts as adultery for legal purposes. This isn’t a passage about forming one flesh.
Before the three of us take anyone else down a rabbit hole, I think you're both confused about Exodus 22:16-17, because the woman in question was not betrothed: "In case a man entices a virgin who is not betrothed, and he lies with her, he shall pay, yea pay her bride-price as a wife for himself. If her father refuses, yea refuses to give her to him, he shall weigh out silver according to the bride-price for virgins." [Concordant Version of the Old Testament]
It says right in :16 that she wasn't betrothed. I agree that the passage isn't about one flesh, even though one-flesh occurred; it's about adultery, because the one-flesh episode occurred outside of an existing agreement with the enticee's father. With many translations, the combination of :16 and :17 confusingly reads such that one could almost interpret :17 to read that, in the event that the father refuses, then he, the father, must pay the bride price to the enticer, but that's just confusion based on translator vagueness. The point I see @Pacman making is that the enticer wasn't necessarily guaranteed to end up possessing the enticee long-term, and it is driven home by The Word emphasizing it by stating that the bride-price must be paid whether the father agrees to the marriage or not.
I don't read this passage as having as its main message defining marriage; on the contrary, I read it as an injunction that one cannot legitimately force a father's hand by seducing his daughter without permission from the father. The seduction does not eliminate the father's power to determine who marries his daughter.
One-flesh-definition occurs here, but long-term possession is not guaranteed, and I'm going to have to put this in my pipe and smoke it, because it may, even just by itself, indicate that one-flesh is insufficient on its own to define marriage. But . . . on the other hand . . . there is this: does this passage or any other passage assert that, if the man pays the bride price, the woman's father refuses to recognize the union, but the man and woman then refuse to dissolve their union in defiance of the father's refusal to recognize it, going on to live the rest of their lives together, would this continued union be considered fornication and/or adultery?
Another way of asking this question is to ask whether it may be reasonable interpretation to consider Exodus 22:16-17 as having as its main thrust assertion of the essential nature of properly paying for a bride before possessing her?