When the Reverend Martin Madan wrote the first volume of his seminal work, Thelypthora, he took the time to consider the case of Boaz and the kinsman who was nearer to Ruth than he was. This is recorded in Ruth chapter four. He offers a succinct and common-sense analysis of why the kinsman refused. Let’s read Ruth 4:1-12 (NASB).

Now Boaz went up to the gate and sat down there, and behold, the close relative of whom Boaz spoke was passing by, so he said, “Turn aside, friend, sit down here.” And he turned aside and sat down. 2 He took ten men of the elders of the city and said, “Sit down here.” So they sat down. 3 Then he said to the closest relative, “Naomi, who has come back from the land of Moab, has to sell the piece of land which belonged to our brother Elimelech. 4 So I thought to inform you, saying, ‘Buy it before those who are sitting here, and before the elders of my people. If you will redeem it, redeem it; but if not, tell me that I may know; for there is no one but you to redeem it, and I am after you.’” And he said, “I will redeem it.” 5 Then Boaz said, “On the day you buy the field from the hand of Naomi, you must also acquire Ruth the Moabitess, the widow of the deceased, in order to raise up the name of the deceased on his inheritance.” 6 The closest relative said, “I cannot redeem it for myself, because I would jeopardize my own inheritance. Redeem it for yourself; you may have my right of redemption, for I cannot redeem it.” 7 Now this was the custom in former times in Israel concerning the redemption and the exchange of land to confirm any matter: a man removed his sandal and gave it to another; and this was the manner of attestation in Israel. 8 So the closest relative said to Boaz, “Buy it for yourself.” And he removed his sandal. 9 Then Boaz said to the elders and all the people, “You are witnesses today that I have bought from the hand of Naomi all that belonged to Elimelech and all that belonged to Chilion and Mahlon. 10 Moreover, I have acquired Ruth the Moabitess, the widow of Mahlon, to be my wife in order to raise up the name of the deceased on his inheritance, so that the name of the deceased will not be cut off from his brothers or from the court of his birth place; you are witnesses today.” 11 All the people who were in the court, and the elders, said, “We are witnesses. May the Lord make the woman who is coming into your home like Rachel and Leah, both of whom built the house of Israel; and may you achieve wealth in Ephrathah and become famous in Bethlehem. 12 Moreover, may your house be like the house of Perez whom Tamar bore to Judah, through the offspring which the Lord will give you by this young woman.”
Ruth 4:1-12 NASB
Even in Madan’s time, theologians who were monogamy-only were inventing reasons why the nearer kinsman chose to refuse, instead of accepting the man’s reason at face-value.
Note: the original text is written in Middle English with the f often standing for the s character. I am presenting the quotation from pages 251-252 in 21st century formatting.
But the learned Dean, in order to overthrow all the Bishop’s reasoning on the subject, observes from Selden’s Ux. Heb. that “the Chaldee paraphrast, the Midrash, and Josephus agree, that this was the reason why Mahlon’s next kinsman refused to redeem Ruth, his widow, viz. – Because it was not lawful for him to marry her, having a wife of his own.” That people should invent reasons for men’s actions, where none are given, is not so surprising as overlooking the reasons that are given, and substituting others which do not appear to be somuch as thought of by the parties themselves. This is the case here – Mahlon’s next kinsman is applied to, as by law he ought to have been, to buy Mahlon’s inheritance, and to marry Ruth, his widow: his answer is neither more nor less than this – “I cannot redeem it for “myself, lest I mar my own inheritance.” – How these words relate to the lawfulness or unlawfulness of the matter, was reserved for the ingenuity of modern interpreters to make out – that they may, and most probably do, relate to the expediency or inexpediency of such a step, the kinsman’s present circumstances considered, may be easily inferred from the reason given. The kinsman might be married, perhaps have many children, and but a small provision for them – therefore, when he hears of not only disbursing the redemption-price for Mahlon’s parcel of land, but that this could not be done, without marrying a very poor young woman, by whom he might have another numerous family of children, which he could not maintain, educate or provide for, out of the small parcel of land which was Elimelech’s, but must diminish his own inheritance of which he was possessed, to the damage of the other family, he prudently declines the kinsman’s part – less, said he, I mar my own inheritance. Such a sense as this the words will most certainly bear; but as to their meaning that the man would not marry Ruth – “because it was not lawful for him to marry her, having a wife of his own” – it is a conceit, fetched even farther than, one would think, the utmost unfairness of prejudice itself could reach.”
Madan’s remarks on the reasoning of the kinsman as recorded in Ruth 4:6 makes me think of Yeshua’s commentary on the costs of discipleship as recoded in Luke 14.
28 For which one of you, when he wants to build a tower, does not first sit down and calculate the cost to see if he has enough to complete it? 29 Otherwise, when he has laid a foundation and is not able to finish, all who observe it begin to ridicule him, 30 saying, ‘This man began to build and was not able to finish.’ 31 Or what king, when he sets out to meet another king in battle, will not first sit down and consider whether he is strong enough with ten thousand men to encounter the one coming against him with twenty thousand?
Luke 14:28-31 NASB
Indeed, a wise man must consider the costs of supporting more than one wife with her children. If he cannot do it, he should not marry, which also applies to whether or not to marrying the first wife in the first place!