1 Marriage is God's best plan for a man and a woman together 2 Divorce should be treated like war, sometimes necessary, always evil Situations do arise when this life-long covenant is violated through unfaithfulness of some kind and the best option is divorce. This was a protection for women in that 1st Century culture where they were still regarded as the property of their husbands able to be discarded almost at a whim as if changing your marriage partner was just a matter of moving house or getting a new car. But we should not take these few words of the Lord answering the negative questioning of the Pharisees as conclusive for all cases of marriage breakdown. If someone has been deserted for years their case may not strictly fit the examples which we have in the Bible and yet be reasonable grounds for divorce. We have the same problem with other teachings of Jesus how to live by them in a fallen world.
War is an example.
Many Christians have read the Sermon on the Mount and become pacifists, inspired by Jesus' teaching on non violence Other Christians have considered the whole teaching of the Bible and decided that there are times when violence must be met by violence as a necessary evil to prevent a greater evil. That was flippant because the single state at that time was unusual and frowned upon. To be a eunuch, to be physically incapable of marriage was to be a second class person, to be denied the priesthood in the Old Testament. And to have chosen not to marry, as Jesus himself had, was unusual. Jesus here sees the single state as a gift from God, not one that can be accepted by everyone.