Disregard for the sanctity of marriage is nothing new. It was a problem in the days of Jesus as evidenced in Matthew 19:1-12. Four centuries prior it was a problem for Israel (Mal 2:13-16), and more than 1000 years before that, Moses had to address the problem in the giving of the law (Deut 24). Though never mentioned in Scripture prior to Moses’ Law, we can assume that such was also the case in the previous millennia when the world became utterly corrupt (cf Gen 6:5).
In Matthew 19:4-5, Jesus goes back to the Creation in reminding men what was God’s original plan for marriage, namely, one man and one woman for life until separated by death. While Adam spoke of the two becoming one flesh, no statement is made concerning the authority of the joining until Jesus’ statement in verse 6, “Therefore what God has joined together, let not man separate.”
The fundamental principle of marriage is that of oneness. That is, two individuals, becoming as one in the Divine institution and covenant of marriage. Many, if not all, of the questions about what God recognizes as proper marriage can be answered using this principle.
The Bible uses accommodative language to speak of or recognize things that happen in reality, (e.g., marry, married, etc.). However, it is a far different thing to say that God approves of that reality.
Our text contains just such a case. In Matthew 19:9, Jesus clearly recognizes that two people who have been joined together by God can divorce, and that one or both can marry another. But those who divorce their mates for reasons other than fornication are said to commit adultery in those cases.
God recognizes that a marriage has taken place, but by implication denies its right to exist by identifying that relationship as constituting adultery.
A similar case is that of Herod in Mark 6:17-18. The text says Herod had married Herodias, but John declared, “It is not lawful for you to have your brother’s wife.”
Thus, in determining the validity of second (or third, or…) marriages, the only question that needs to be addressed is, “Did God join these two together?” If the answer is no, that relationship cannot continue with Divine approval.