An interesting piece on divorce. Chapter 24 states that the ground for divorce are a man "finding some impurity" in his wife. "Impurity" in this context seems to mean sexual sin. It can't be ritual impurity, since this can be done away with and in many cases isn't blame-worthy. If it is sexual sin, then it is adultery, since that's really the only sexual sin that a married woman can commit.
Hence, Jesus is not actually intensifying the OT as many exegetes seem to suggest, but rather interpreting what "impurity" means. The question of "why did Moses allow us to divorce our wives" and Jesus response because "your hearts were hard" is really not a problem to this interpretation. Non-fallen human (ones without hard hearts) wouldn't have divorce for any reason, much less adultery, since they wouldn't sin.
This is a major point for the Reformation understanding of Jesus' mission over against the Latin tradition's interpretation. Beginning with Tertuallian, it was claimed that Jesus came to give a better and tougher law than Moses. Later this was softened into the distinction between normal, regular law for the laity and "evangelical counsels" for mendicants.
The Reformers (both Lutheran and Reformed) emphasized that Jesus did not come to give a better law, but merely clarify the existing natural law as summarized by the Ten Commandments. Even the prohibitions against looking at persons of the opposite sex with lust or anger being as bad as murder (in the Sermon on the Mount) are in fact found in the Ten Commandment's prohibitions against coveting and in God's admonish throughout the OT to love one's neighbor.
On a side note: When I read the passages in the Gospels where Jesus argues with the Pharisees about the law, I'm always impressed by his patience with his creatures. He actually argues with them, and patiently refutes their claims. He could very well say: "Hey, who do you think Moses was talking to on Mt. Sinai? It was me! I should know what I intended to say! So knock it off!" But he of course doesn't.