From The Gospel of Mark by Mary Healy, comnenting on Mark 10:9-12:
Jesus concludes with his own solemn injunction: what God has joined together, no human being must separate. He thereby confirms what Genesis already implied: the union of husband and wife is no mere human convention but a bond made by God himself (see Mal 2:14–16). No human being is authorized to dissolve that bond once it has been made.
It is no wonder that the disciples, as often happens, find it difficult to digest the radical change Jesus has just instituted (see Matt 19:10). On his own authority Jesus has just taken away a concession given in the law of Moses. Why would he set this stricter standard? Surely it is not to make life more difficult for his followers. Rather, it is because through his cross and resurrection he is now giving them a new power to live according to God’s original plan for human love. They can no longer settle for less. Once again, in a private indoor setting, the disciples ask Jesus to explain himself (see Mark 4:10; 7:17; 9:28). For Mark’s first readers, in the house probably called to mind the Church, which gathered in homes just as the first disciples had often gathered around Jesus in a house (1:29; 2:15; 7:24; 9:33). Jesus unpacks the implications of his teaching by declaring that remarriage after divorce is not permissible. Whoever divorces his wife and marries another commits adultery against her. This statement is radical in two ways. First, it affirms the indissolubility of marriage, a teaching that is as challenging and countercultural today as it was then. Second, it recognizes adultery as an offense that can be committed against a wife. Jewish law and custom had viewed adultery as an offense against a man, whose wife was considered in some sense his property (see Exod 20:17). Jesus acknowledges the total equality of man and woman, and the mutual belonging of husband and wife in marriage. The final statement reflects the situation in the Roman world where women had a legal right to divorce, and affirms that women are equally responsible for upholding the permanence of the marriage bond.