In this passage, the Pharisees tempted Jesus with a question concerning divorce. They figured that he would be against divorce, but they knew that Moses allowed it. So, they were using this question to trip him up with the law. Of course, their temptation didn’t work. Instead, he gave them a doctrinal statement regarding one of three reasons why a person could remarry.
The question was, “Is it lawful for a man to put away his wife for every cause?” The answer is, “No.” Jesus explained in verses 4-6 that marriage brings a man and a woman together as one flesh and that no man should put asunder what God has joined. This principle came originally from Adam and Eve. That’s why Jesus quoted Gen 2:24.
Eve actually came from Adam’s body and that’s why Adam said, “This is now bone of my bones, and flesh of my flesh,” (Gen 2:23). The picture is that of Jesus and the Church according to Eph 5:30-32, where Paul says, in the context of Gen 2:24, “we are members of his body, of his flesh, and of his bones.” Obviously, since we are one flesh with Jesus Christ eternally and nothing can sever our position there, then God doesn’t want anything to sever the “one flesh” of a marriage either.
In verses 7 and 8, therefore, the Pharisees wanted to know why Moses allowed divorce (Deut 24:1). The answer was simply, “because of the hardness of your hearts.” In other words, this was not God’s desire, it was allowed only because of their hard-heartedness.
In verse 9, Jesus narrowed the reason for divorce from “for every cause,” (in verse 3) to “fornication.” The reason he allowed a divorce in the case of fornication is that fornication creates a new union of flesh with flesh. Paul said in 1 Cor 6:16, “What? Know ye not that he which is joined to an harlot is one body? For two, saith he, shall be one flesh.” This is the classic case of “adultery” (Matt 19:9) and gives the offended spouse the right to divorce the adulterous spouse (although divorce is not commanded, but rather allowed). If the divorced spouse in this case were to remarry, there would be no adultery.
In actuality there are two other reasons why a spouse could remarry. The second one is obvious. Men and women can remarry after their spouses die (1 Cor 7:39; Rom 7:1-4).
Furthermore, they can remarry if they are deserted by their spouses, although Paul recommends that they stay single (1 Cor 7:11-16, 27-28). Desertion obviously breaks the marriage and leaves the remaining spouse without a husband or wife, as they case may be (Hos 2:1-2). In that case a spouse is free to remarry (1 Cor 7:15, 28) without committing adultery.
We recognize that many preachers have taught on remarriage differently based on Rom 7:1-4, but that passage has nothing to do with divorce. Paul uses marriage there to illustrate what happens to the soul when a person gets saved. The soul (pictured as the wife) is free to marry Jesus because the body (pictured as the husband) is dead. For more on this see the question in the question archives If I have been divorced must I remain unmarried?
In verses 10-12, the disciples decide, after hearing Jesus’ limitation on divorce, that it would just be better for men not to marry at all. But Jesus responded that all men couldn’t do that unless they were born eunuchs, made eunuchs (like Shadrach, Meshach, and Abednego) or had received the ability to make themselves eunuchs for the kingdom’s sake. Paul called this ability a “proper gift from God,” (1 Cor 7:7) and wisely added “it is better to marry than to burn,” (1 Cor 7:9).