Mark 10

2 Jesus had undoubtedly spoke on this topic earlier in His ministry. Apparently knowing that Jesus was against divorce, they wanted to trap Him into contradicting the Law of Moses.
3 Jesus, already knowing their intention asks them to recite what the Law said.
4 Deu 24:1 allows divorce in cases of sexual misconduct (apparently, adultery did not always end in stoning). However, the Pharisees, ironically strict in other legal matter, expanded the application of this verse. Apparently, divorces could occur for almost any reason. Wycliffe credits this interpretation to Hillel.
5 Jesus replies that God gave Moses this law because of man's sinful nature. Divorces should not occur (neither should adultery). In addition, the law was given to restrict the conditions of divorce, not to give unbounded license for divorce.
6 To justify His position, Jesus returns to the historical account of creation in Genesis. In marriage, a man and woman leave their respective families and join to form a new family unit.
8 In the mystery of marriage, the man and woman become as one.
9 Thus, to separate the married couple would be unnatural, painful, and harmful. Anyone who has witnessed a divorce knows the damage divorce causes to husband, wife, children, and extended family.
10 The disciples, brought up under the interpretations of the Pharisees sought further clarification on this topic.
11 Jesus builds on Deu 24:2 to show that divorce tends to lead to compounded sin as divorced couples remarry. First, a woman divorced against her will would have limited options. Her birth family would not necessarily receive her back. Her divorced family would most likely reject her. She would need to find a new husband to support her. Second, the husband who divorces his wife for the sole purpose of marrying another commits multiple sins.

The point of the adultery angle is to show that in God's eyes divorce and remarriage offends Him no matter what the reason for divorce was (He considers the original marriage the only valid one). It is also clear that the person who initiates the divorce bears the responsibility. It appears that sexual immorality is the only valid reason for divorce because the offender has already broken the marriage with adultery.

17 See commentary on Mat 19:16-22.