Genesis 21:11 - Joseph Benson’s Commentary on the Old and New Testaments

The thing was very grievous in Abraham's sight Because of his affection to his son, and God's promise concerning him. He who, at God's command, which he was bound to obey, afterward so cheerfully gave up Isaac, was not so ready to part with Ishmael, to gratify the passion of an angry woman. And probably he would have denied her desire, if God had not interposed. It is remarkable that it is not said the thing was grievous because of his wife; probably he hardly considered Hagar as properly his wife: or, at least, had not the affection for her a man ought to have for his wife. Hereby we may learn the excellence of God's institutions, who appointed only one woman for one man, that each might, under God, have the entire interest in the other's affections; and we may observe the evil of men's inventions which brought polygamy into the world, whereby a man's affections are divided into several and contrary streams. But probably it grieved Abraham that Ishmael had given such provocation, as well as that Sarah insisted on such a punishment.