In Matthew 19:3-9, the Lord Jesus is not speaking about polygamy. Rather, He is only answering a question about divorce. Indeed, the entire passage is about divorce, not polygamy.

"The Pharisees also came unto him, tempting him, and saying unto him, Is it lawful for a man to put away his wife for every cause? And he answered and said unto them, Have ye not read, that he which made them at the beginning made them male and female, And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh? Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder. They say unto him, Why did Moses then command to give a writing of divorcement, and to put her away? He saith unto them, Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so. And I say unto you, Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery." Matthew 19:3-9.

For clarity, readers may want to also see:

• Adultery - "woman that breaketh wedlock"
• One Flesh - written by polygamous Moses

Indeed, in Matthew 19:8-9, Jesus simply repeats the Deuteronomy 24:1 "as it had been in the beginning", that God has always been against divorce of what He joined together. In Matthew 19:3, the Pharisees were asking about "every" reason for divorcing, but Jesus returned back with the only one allowed reason (the woman's "fornication / uncleanness"), as per Deuteronomy 24:1.

Polygamy is about marriage, whereas divorce is about breaking up marriage. Accordingly, Jesus was not speaking against pro-marriage polygamy in Matthew 19. He was instead addressing marriage-destruction of divorce. It was about divorce, not polygamy.

Also See:

   Remember Moses wrote it ©   (Genesis 2:24)

   Adultery

   One Flesh