Ezekiel 16:1-15, 60, 63 or Ezekiel 16:59-63
The question that the Pharisees and Jesus are discussing in the Gospel today is very interesting. Nowhere in the Law does God say that a man can divorce his wife, but several times the Law mentions divorce and sets down certain rules that were not to be broken. The Pharisees read the Law and decided that God would not set down rules about divorce unless it was okay to get divorced.
Yet something was still bothering them: getting married means making a promise to be with a person and take care of them. Normally, it is unacceptable to break promises, especially a promise as solemn and public as the marriage vows. So the Pharisees wondered what reasons justified breaking the promise. Indeed, the Scripture says that they were testing Jesus. Perhaps they had their own list of reasons and wanted to see if Jesus could come up with as good a list.
Instead, Jesus explains that their initial logic was faulty. Divorce is mentioned in the Old Law, but it was one of the imperfections of the Old Law. The Old Law was true as far as it goes, but it was coping with the how resistant the human heart is to the Truth. God does not allow divorce, but a commandment forbidding divorce would have been too much for the Israelites to put up with. So God set down certain rules, saying, if you are going to get divorced, here are some absolute minimum standards of conduct: a man who divorces his wife should not keep her as a slave, and, if a man is free to remarry, so is his wife.
What changed in the 1500 years since Moses? Human nature did not change. True, 1500 years of living the Old Law had prepared the world to receive the New Law, but the New Law is not just the Old Law with more difficult rules. The New Law is the Holy Spirit. By the power of the Holy Spirit who dwells within us, we do not have to settle for the best we can handle, our standard now is perfection. The Holy Spirit, God himself, lives in our hearts; he is a law written on our hearts. If we let him, he can make us perfect. A life-long marriage between two sinful human beings (and we are all sinful) is impossible without the Holy Spirit, but with God all things are possible.