John Calvin Commentary Genesis 2:24

"Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." — Genesis 2:24 (ASV)

Therefore shall a man leave. It is debated whether Moses here introduces God as speaking, or continues Adam's discourse, or, indeed, has added this by virtue of his office as teacher, speaking for himself. The last of these is what I most approve. Therefore, after he has historically related what God had done, he also demonstrates the purpose of the divine institution.

The sum of the whole is that, among the duties pertaining to human society, this is the principal, and as it were the most sacred: that a man should cleave to his wife. And he amplifies this by a further comparison, that the husband should prefer his wife to his father.

But the father is said to be left not because marriage severs sons from their fathers, or dispenses with other ties of nature, for in this way God would be acting contrary to Himself. However, while the son's devotion to his father is to be most assiduously cultivated and should in itself be considered inviolable and sacred, yet Moses speaks of marriage in such a way as to show that it is less permissible to desert a wife than parents.

Therefore, those who, for slight causes, rashly permit divorces, violate, in this single act, all the laws of nature, and reduce them to nothing. If we consider it a matter of conscience not to separate a father from his son, it is an even greater wickedness to dissolve the bond that God has preferred above all others.

They shall be one flesh. Although the ancient Latin interpreter translated the passage as ‘in one flesh,’ the Greek interpreters expressed it more forcibly: ‘They two shall be into one flesh,’ and Christ quotes the passage this way in Matthew 19:5. But though no mention is made of two here, the meaning is unambiguous. For Moses had not said that God assigned many wives, but only one to one man; and in the general direction given, he had used ‘wife’ in the singular. It remains, therefore, that the conjugal bond subsists between two persons only, from which it easily appears that nothing is less in accordance with the divine institution than polygamy.

Now, when Christ, in censuring the voluntary divorces of the Jews, gives as His reason that it was not so in the beginning (Matthew 19:5), He certainly commands this institution to be observed as a perpetual rule of conduct. To the same point, Malachi also recalls the Jews of his own time: Did he not make them one from the beginning? And yet the Spirit was abounding in him (Malachi 2:15).

Therefore, there is no doubt that polygamy is a corruption of legitimate marriage.