Let's back up a view the full passage here to get a better grasp of what Christ is teaching. This passage is within Mark 10:1-12 and has a parallel in Matthew 19:1-12. What reason does Christ give that trivial divorce is wrong and that remarriage after such a divorce is adultery? Well, in v.6-9 Christ tells us exactly the reason why: because only two people can be united into one flesh (marriage). These two people can only be united to each other for the entirety of their lives. Further this is a callback to Genesis 2:24 where the Biblical framework for marriage is established in the story of Adam and Eve. This stands as a strong basis, from the mouth of Christ, as grounds against polygamy.
Yes, many Old Testament heroes had more than one wife, but just because the Bible records it happening does not mean the Bible is supporting something. The Bible only supports things that it outright says are good. Just because people in the past like Martin Luther didn't know how to deal with these things does not mean that the Bible supports it.
Further, in the Bible, God marries one united nation of Israel. In Ezekiel 16:1-8, we see a fascinating image of the birth of the nation of Israel being separated out from the nations around her. God said that His marriage to Israel began when He entered into covenant with her, which the latest covenant was at Mt. Sinai in Exodus. The reason that Jeremiah 3 uses the language of sisters is because the one united nation that God married had split into two sister nations. This is not to say God was married to two different nations, but rather that the Northern and Southern kingdoms considered themselves as separate. In Jeremiah 3 God uses the Northern kingdom as a warning for the Southern kingdom, and is not implying that God is married to multiple nations.
Next let's move to Christ and His Bride. Your insinuation that Christ is somehow extremely polygamous is over and beyond a misreading of the Scriptures. Yes, the Bride of Christ is comprised of the nations of the world. However, the Bible is clear that this marker of "nations" is not an appropriate way to consider the Bride of Christ. In Galatians 3:28 the marker of "nations" is removed for those who make up the Bride of Christ and are all considered as one. Again, if we flip to 1 Corinthians 12, Romans 12, Colossians 1, and anywhere else the Bible talks of the body of Christ we see it is one unified singular body. We know that the Bride and body are synonyms given in the Bible for the Church. We can clearly see that Christ is not married to multiple brides, but one Bride.
Finally, the picture given in the New Testament for marriage is that of Christ and His Bride. In Ephesians 5:22-33, Paul paints a picture of marriage: Christ and the Church. Just as there is one Christ and one Church, likewise there is one husband and one wife.
Altogether, the Bible is beyond clear that the condoned marriage is one of two people united together and remain faithful to one another for their whole lives.
