Deuteronomy 25:7

“And the man like not to take his brother’s wife, etc.] The provision here made by this law, when this was the case, is such as did not take place before it became a law; for then Onan would have taken the advantage of it, and refused marrying his brother’s wife, which it is plain was not agreeable to him, (<013809>Genesis 38:9); as many do now on one account or another. Leo of Modena f453 says, “it was anciently accounted the more laudable thing to take her, than to release her; but now the corruption of the times, and the hardness of men’s hearts, are such, as that they only look after worldly ends, either of riches, or of the beauty of the woman; so that there are very few that in this…”

“The law of levirate marriage. The law on this subject is not unique to the Jews, but is found (see Genesis 38:8) in all essential respects the same among various Oriental nations, ancient and modern. The rules in these verses, like those upon divorce, do but incorporate existing immemorial usages, and introduce various wise and politic limitations and mitigations of them. The root of the obligation here imposed upon the brother of the deceased husband lies in the primitive idea of childlessness being a great calamity (compare Genesis 16:4; and note), and extinction of name and family one of the greatest that could happen (compare Deuteronomy 9:14; Psalms 109:12-15). To avert this the…”