Thank you for explaining your view on the matter as this helps me direct my response. There are two main problems that I see with your reasoning about the morality of polygamy.
The first is that the commandments of the Old Testament do makeup the Law, but the OT is incomplete, being completed only by Jesus Christ in the NT. So, to make an argument about the morality of a subject that is based only on the OT, while not taking into account the NT, is problematic since the NT clearly talks about this matter. It is true that Jesus (Matt 19:3-9) is responding to a question about divorce. However, he clearly says (citing Genesis), “So they are no longer two but one flesh.” He says “two”. I know you disagree with my interpretation of this text, but this leads to the second point…
The second is that your reasoning doesn’t appear to include the magisterial teaching of the Church. God created in the beginning; He gave the Law to the Israelites; then came the Incarnation where God became man, fulfilling the Law that came before; with Christ’s death, the Church was born and carries on the teaching and authority that Christ came with. Therefore, the Church’s teaching is the teaching of Christ. It is the Church who has determined that the words of Jesus cited above mean that polygamy is forbidden. It is true that there doesn’t seem to be a lot of direct magisterial attention paid to it, but it is still mentioned (for example in the 1880 encyclical of Pope Leo XIII, Arcanum, on Christian Marriage). Now, if you aren’t Christian or do not recognize the Church as having any moral authority, there isn’t much more I can say here: I am not interested in interpretations of history, but the real truth of the matter, and there isn’t any truth apart from Truth Itself.
I have one thing to say further on this. 1 Corinthians 7:38: “So that he who marries his betrothed does well; and he who refrains from marriage will do better.” Marriage is good, but virginity is better. We are moving in the wrong direction if we think that our happiness will increase if we increase the number of our wives.