Genesis 16:1

We have here the marriage of Abram to Hagar, who was his secondary wife. Herein, though he may be excused, he cannot be justified; for from the beginning it was not so: and when it was so, it seems to have proceeded from an irregular desire to build up their families, for the speedier peopling of the world. But now we must not do so? Christ has reduced this matter to the first institution, and makes the marriage union to be between one man and one woman only.