In the verse it states
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.
This raises the question: What if the living brother already has a wife? Does he get another one if his brother dies, and he had a wife?
When a woman gets married, she leaves her own family (even in modern ceremonies the bride is "given away" by her father).
If her husband dies and they have no children, she no longer has any real relationship to the family she is living with, but neither is it appropriate for her to return to her old family. If they have a child though, she would be related to them, because she and they are all related to that child.
As described in Deuteronomy 25, to avoid this problem, the eldest brother of the dead husband would "marry" her, producing a child as if it were the original husband's. If there were no brother, then the next in line (e.g. her father in law) would assume the duty.
A famous and complicated case of this is recorded in Genesis 38. Jacob's son Judah had five sons, the oldest two being Er and Onan. Er married Tamar, and then died before fathering any children.
Genesis 38 (NLT)
8 Then Judah said to Er’s brother Onan, “Go and marry Tamar, as our law requires of the brother of a man who has died. You must produce an heir for your brother.”
Notice that this wasn't a case of Onan deciding to marry a second wife; in fact he didn't even want to marry her:
9 But Onan was not willing to have a child who would not be his own heir. So whenever he had intercourse with his brother’s wife, he spilled the semen on the ground. This prevented her from having a child who would belong to his brother. 10 But the LORD considered it evil for Onan to deny a child to his dead brother. So the LORD took Onan’s life, too.
Once the widow had produced a child, she was guaranteed a place in the family because of the genetic connection through that child. It might be convenient for her to continue living as a second wife, but that wasn't a requirement; she could just live as one of the women in the extended family.
So yes, technically this was a case of polygamy, but a case of obligation, not of free choice.
(For those not familiar with the story: Judah sends Tamar back to her old family temporarily until the third brother is old enough, but never does send for her, so she poses as a prostitute and tricks Judah into giving her a child.)
Deuteronomy 25:5 has nothing to do with polygamy. Its purpose is clearly stated in Deuteronomy 25:6:
The first son she bears shall carry on the name of the dead brother so that his name will not be blotted out from Israel.
The Lord commanded the Israelites to keep their own land. In Number ch36, the account of Zelophehad's daughters led to the command that land inheritance shall not pass from one tribe to another (Number 36:9). A family's land could not be permanently sold; even if a family became poor, the land must be redeemable in the future. (Leviticus 25:23-28)
In Christianity, land inheritance in ancient Israel is seen as a foreshadowing of spiritual truths: it represents the Promise Land of God to be inherited by the elect, a place of ultimate rest and fulfillment found in Christ.
Hebrews 4:8-11 states:
8 For if Joshua had given them rest, God would not have spoken later about another day. 9 There remains, then, a Sabbath-rest for the people of God; 10 for anyone who enters God’s rest also rests from their works, just as God did from his. 11 Let us, therefore, make every effort to enter that rest, so that no one will perish by following their example of disobedience. (NIV)
Deuteronomy 25:5 addresses to a specific situation: when a husband dies childless, the widow could have a son with his brother to inherit the deceased's land, thus fulfilling the Lord's command.
While polygamy could occur, it was not always the norm. Typically, the eldest son would marry first and have children before the younger sons married. If the eldest son had children, his wife, upon becoming a widow, would not need to marry the younger brother.
The Lord does not encourage polygamy. However, in ancient patriarchal society, it could provide protection and provision for women who might otherwise be vulnerable. Compared to the importance of inheritance, polygamy becomes a lesser issue in the time before Christ.
The principle found in Deuteronomy 25:5 is about brother-in-law, or levirate, marriage. (Additional information can be found in the topic "Brother-in-law Marriage" in the Insight on the Scriptures)
The topic of "Marriage" in the Insight on the Scriptures gives several statements that help us see Jehovah's viewpoint on polygamy:
Since God’s original standard for mankind was for the husband and wife to become one flesh, polygamy was not intended, and it is prohibited in the Christian congregation. (1Timothy 3:2, 12; Titus 1:5, 6) . . . As was the case with divorce, polygamy, while not God’s original arrangement, was tolerated until the time of the Christian congregation. . . . Concubinage and polygamy no doubt enabled the Israelites to increase at a much faster rate, and therefore, while God did not establish these arrangements but only allowed and regulated them, they served some purpose at the time. (Exodus 1:7)
So while the Mosaic Law's proscription of brother-in-law marriage did lead to polygamy it was not what Jehovah God originally intended and, as commented by Stephen Disraeli, polygamy was tolerated.
No, not for Christians. This was part of the Old Testament "Pentateuch", or the five books of the Law of Moses, aka the Mosaic Law. Even modern-day Jews do not practice this. The Old Testament provided background on how things were (and still are) culturally worldwide, not necessarily on how things 'ought to be. A lot of times when the Scripture says the phrase "you shall", it isn't prescriptive, but descriptive. It's saying "you shall do this as a result of doing this", not "you shall do this because it's something I told you to do.". It's like saying "if you drink a whole handle of liquor, you shall become drunk". Variable X begets the result of Y.
Let us take the ten commandments for instance, or even "the Great Commission" in the New Testament, in the Book of Acts. Look at the prefix in the words "commandments" and "commission". The prefix is the same, it's "co", meaning you do something alongside someone at the same time, like a child "CO-operates" with their parents' wishes, demands, or requests, we cooperate with Jesus Christ as a result of walking in His Spirit, the Holy Ghost, and we keep his commandments as a result, for He is God.
In the context of the the ten commandments, and in the context of our entire walk with the Lord Jesus Christ throughout our earthly life, we are the clay in the potter's hands. As a result of walking in the Spirit, God molds us into a person with a different nature than the old person who had a nature that caused them to break the commandments. But, as we walk in the Spirit and in our new born-again nature, we progressively and over time, turn away more and more away from the things God told us we shall not do, not because we are ticking off boxes in a mental checklist of things not to do, but as a default result of our new nature, we just don't do those things anymore.
Hope this all makes sense. I know this probably does not directly answer your question, but I hope that this provides a new lens to look at your question through that can hopefully further illuminate answers and insight.
