Judaism, Christianity, and Islam all regarded polygamy as legal. I don’t see how you can say that God was opposed to polygamy.
I didn’t. That was someone else.
I can easily say that it is opposed in Christianity:
Gospel of Matthew Chapter 19 (note the nouns being singular, not plural):
3. Some Pharisees approached him, and tested him,* saying, “Is it lawful for a man to divorce his wife for any cause whatever?”
4. He said in reply, “Have you not read that from the beginning the Creator ‘made them male and female’
5. and said, ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh’?
6. So they are no longer two, but one flesh. Therefore, what God has joined together, no human being must separate.”
7. They said to him, “Then why did Moses command that the man give the woman a bill of divorce and dismiss [her]?”
8 He said to them, “Because of the hardness of your hearts Moses allowed you to divorce your wives, but from the beginning it was not so.
9. I say to you,* whoever divorces his wife (unless the marriage is unlawful) and marries another commits adultery.”
10. [His] disciples said to him, “If that is the case of a man with his wife, it is better not to marry.”
11. He answered, “Not all can accept [this] word,* but only those to whom that is granted.
12. Some are incapable of marriage because they were born so; some, because they were made so by others; some, because they have renounced marriage* for the sake of the kingdom of heaven. Whoever can accept this ought to accept it.”
Gospel of John, Chapter 4 (the samaritan woman at the well):
16. Jesus said to her, “Go call your husband and come back.”
17. The woman answered and said to him, “I do not have a husband.” Jesus answered her, “You are right in saying, ‘I do not have a husband.’
18. For you have had five husbands, and the one you have now is not your husband. What you have said is true.”