No serious scholar agrees with this defense, and for good reason.
To begin with, Ishmael was Abraham’s begotten son regardless of the nature of
his parentage. More concrete validation of his status as Abraham’s legitimate son is
simply that God recognized him as such, as encountered in Genesis 16:11, 16:15, 17:7,
17:23, 17:25, 21:11. And if God recognized Ishmael as Abraham’s son, who of mankind
dares to disagree?
But man is inclined to argument, so by looking at all angles a person should
recognize that polygamy was an accepted practice according to the laws of the Old
Testament. 69 Examples include Rachel, Leah, and their handmaids (Gen 29 and 30),
Lamech (Gen 4:19), Gideon (Judges 8:30), David (2 Samuel 5:13), and the archetype of
marital plurality, Solomon (1 Kings 11:3). The Oxford Dictionary of the Jewish Religion
notes that polygamy was permitted in the laws of the Old Testament, and was recognized
as legally valid by the rabbis. 70 Encyclopedia Judaica acknowledges the common
practice of polygamy among the upper classes in biblical times. 71 Polygamy was banned
among Ashkenazi Jews in the tenth century, but the practice has persisted among
Sephardi Jews. 72 73 Even in Israel, the chief rabbis officially banned the practice only as
recently as 1950, and considering the thousands of years it took to rewrite Mosaic Law,
we have good reason to suspect the above rulings were motivated more by politics than
by religion. 74
So what should we understand when Genesis 16:3 relates, “Sarai, Abram’s wife,
took Hagar the Egyptian, her slavegirl, and gave her to her husband Abram as a wife”
(italics mine)? Polygamy may offend Western sensitivities, be that as it may. The point is
that according to the laws of Abraham’s time, Ishmael was a legitimate child.
But purely for the sake of argument, let’s just forget all that (as many do) and say
that Hagar was Abraham’s concubine. Even that claim has an answer. According to Old
Testament Law, concubines were legally permitted, and their offspring had equal rights.