They Become One Flesh: Joy of Text 1:12 by Rabbi Dov Linzer

Hence a man leaves his father and mother and clings to his wife, so that they become one flesh.

לבשר אחד ONE FLESH — Both parents are united in the child.

“Therefore a man shall leave his father and his mother and cleave to his wife...” The holy spirit says this to forbid I chest to the descendants of Noah. “'And they shall be one flesh'- the child is formed by the two of them and thus their flesh is made one', so Rashi. This is without reason, for domesticated and wild animals also become one flesh through their children. In my view animals do not cleave to their sexual partners; the male approaches any female it finds and then leaves. For this reason the text says since the female of the adam was a bone of his bones and flesh of his flesh, he cleaved to her and she was in his bosom like his own flesh and he desired her to be constantly with him. As this was so with Adam so was his nature transmitted to his progeny, to make the males, cleaving to their females, leaving their fathers and mothers and regarding their wives as if they were one flesh with them. Similarly, “you are our brother, our flesh (gen.37:27), “to the meat of ones flesh (Lev.18:6), those close in family are called “meat of flesh” and so will leave the “meat” of his father and his mother and their closeness and see that his wife is closer to him than they are.

And the woman is called she’er of the husband… and that is based on the verse, “And he shall cleave to his wife and they shall be one flesh” (Gen. 12:24). Thus, sh’era means the closeness of her flesh, and ksuta means her bedclothes, as it says, “Because that is his only ksut– garment – with what shall he sleep?” (Deut. 22:26). And onatah refers to time, that he shall come onto her at the time of lovemaking…
The sense of the verse if that if he takes another wide, the closeness of her flesh and her bedclothes and her time of lovemaking he should not withhold from her, because that is the statute of women (lit. daughters). And the reason for this prohibition is that the new wife should not be sitting on a honorable bed, where they will be there as one flesh, and the first wife will be treated as a concubine, sleeping with him only casually (lit. by chance) and on the floor, like one who comes onto a prostitute. Thus the verse prohibits this. And so our Sages have explained, “She’erah implies closeness of the flesh, viz., that he must not treat her in the manner of the Persians who perform their conjugal duties in their clothes.”

This is a correct explanation, because it is the way of Scripture in all places to refer to matters of sex with clean and short terms, and thus it says with these matters obliquely: she’era, ksuta, vi’onatah, to refer to the three matters that a man conducts with his wife at the time of their union.