The passage is a decided and just reproof of those frequent violations of the true idea of the marriage state, which had crept in among the Israelites. God was offended; and the prophet gives the reason of it. When God, in the beginning of things, had created man, he separated from him, in the moment of his "deep sleep," a part of his existence And from that which he thus separated, he made the counterpart and completion of humanity in woman. He made one. In the language of the prophet, he had "the residue of the spirit;" and therefore he might have made a greater number. But that perfect conception which he had of a moral constitution of things, and of the elements of moral happiness, did not allow of more than one.
It was necessary, being good and perfect in himself, that he should so create man, as to evolve or develop from his existence, so long as it continued an unperverted existence, the highest possible degree of happiness. But perfect happiness cannot grow on the basis of a divided affection. It is only fullness of love, or love in the highest degree, — a state of mind which seems to be inconsistent with a multitude of objects of love, — that is crowned with fullness of bliss. And besides, that form or arrangement of the domestic constitution which limits the central or highest affection to one, was foreseen to be most favorable, as we should naturally suppose it would be, and as the passage in Malachi implies, to the birth and training of a "godly' seed." Polygamy and concubinage, and still more other systems, which propose a yet wider and more vicious liberty, are obviously inconsistent with that degree of watchful care, and religious instruction, which is necessary in training up a seed or people for God. And I think it cannot be doubted that the perpetuation of a godly seed is one of the objects involved in the constitution of a moral order of beings. Holiness, like sin, has its law of origin, and its line of descent.
— edited from A treatise on Divine Union (1851) Part 7, Chapter 6.