Doc said
The reason you do not see single mothers much in the Bible is because God had already established an order to care for those that found themselves without a husband because of war, famine, or even abandonment. For instance, if a man was killed in a war, it was the responsibility of the brother (or the next of kin to the husband), to take that wife into his own home and treat her as his own wife. In fact, if the responsible party did NOT perform such an action, he could be found condemned.++
This seems to be coming from the Levirate law. So I looked up the actual scripture of the Levirate law (Deuteronomy 25:5-10) This is a law that we polygyny supporters use often to show that there may be a command for polygyny in the bible.
Deu 25:5 If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her.
Deu 25:6 And it shall be, that the firstborn which she beareth shall succeed in the name of his brother which is dead, that his name be not put out of Israel.
Deu 25:7 And if the man like not to take his brother's wife, then let his brother's wife go up to the gate unto the elders, and say, My husband's brother refuseth to raise up unto his brother a name in Israel, he will not perform the duty of my husband's brother.
Deu 25:8 Then the elders of his city shall call him, and speak unto him: and if he stand to it, and say, I like not to take her;
Deu 25:9 Then shall his brother's wife come unto him in the presence of the elders, and loose his shoe from off his foot, and spit in his face, and shall answer and say, So shall it be done unto that man that will not build up his brother's house.
Deu 25:10 And his name shall be called in Israel, The house of him that hath his shoe loosed.
In Tom Shipley's book "Man and Woman in Biblical Law" he states
"Very little attention has been given to this aspect of the levirate by commentators, even though it is manifest that it mandates polygamy when the surviving brother is married. The childless widow is forbidden to marry outside of her dead husband’s family rather than an already married brother-in-law. She must become the second (or third or fourth...) wife of her brother-in-law."
"The widow is forbidden to marry anyone except the next of kin. That the wife is under the jurisdiction of her husband is assumed in the levirate, but that is not the focus of the law. The focus is upon maintaining the family line of the dead brother. The levirate law emphasizes the duty of both the brother-in-law and the widow to raise up a seed in the name of the deceased."
The scripture specifies that the command is only in effect if the brother died without having had a child with the woman. If she had children by the man who died, this law doesn't seem to apply.
I am a supporter of polygyny, but I want us to be accurate. This is not a command from God to care for widows and orphans but a command from God for brothers to protect each other's lineage.
God allows polygyny. Enough said.