Matthew Poole and continuators on Genesis 16:3

3. And Sarai Abrams wife took Hagar her maid, the Egyptian, after Abram had dwelt ten years in the land of Canaan, and gave her to her husband Abram, to be his wife .

(i. e.) His Concubine, or secundary Wife. Polygamy, though it was forbidden by God's first institution, Gen. 2. 24. compared with Mat. 19. 5. and brought into the World by wicked Lamech, yet it was sometimes practised by the Patriarchs, either by God's permission, who could rightly dispence with his own laws when and where he pleased; or by their mistake about the lawfulness of it. As for the present case, it is most evident this action was not the effect of an inordinate lust, but of an earnest desire of having children, and especially of obtaining the blessed and promised seed.