@AbrahamSolomon, it is obvious that when a couple have sex, or get married, they do not become physically glued together for the rest of their lives. That cannot be what "one flesh" means. And, as
@NickF says, it is not what the Hebrew word "one" means either. "echad" means
united.
Nevertheless, the Bible does say that they become "one flesh" (or rather, become "united flesh" if we are to translate echad more accurately). It has to mean something. We are trying to understand what it means, how do this man and woman become united?
The idea that it is metaphorical is the fallback option. We can always just consider it metaphorical if we can't understand how it works. And that's fine.
But the entire starting point of this thread (go back and read the first post again to refresh yourself) is that there may actually be a physical way that the two become "united". Not physically joined together, but united
in substance. They are now made out of the same "flesh", whereas previously they were made out of different "flesh".
If we have a lump of brown clay, and a lump of white clay, they are different substances - different "flesh" we could say.
If we stick the two together, they are now one lump. But we know men and women don't get stuck together like this.
But if we mix the two together, each is now a mixture of brown and white. If we now separated them so we had two lumps again, although there are two, they are now both made out of the same substance, the same "flesh" - both are made out of a mixture of brown and white clay. They are two lumps, but one "flesh". We could say they are united - they are a matching pair.
My proposal is that it is possible (not certain) that this is what the Bible means. When a man and a woman come together physically, something changes in their bodies to make them united. And the microbiome is the key - the microbiome of a man and a woman truly do become one, science tells us that clearly. And the microbiome is actually the majority of cells in the human body.
So science tells us that a man and a woman truly do start out different, but end up to some degree made out of the same substance, the same "flesh".
That might not be what scripture is talking about. It might be metaphorical. And if that doesn't make sense to you, then just stick with considering it to be metaphorical - that is fine. I'm just going beyond that by looking carefully at the findings of modern biology.