Jesus, a good teacher, answers a question with a question. First, Jesus asks his opponents to think about whether religion is about rules. Moses, the giver of the law, is cited as one who allowed a husband to write a certificate of divorce. But that rule was not meant to encourage or sanction divorce, but existed as a concession to the fact that divorce happened. The laws, the rules, were a way to navigate the occasions when that relationship of marriage has broken.
Rules play an important role.
But what I hear Jesus saying is that the laws, the rules, are not the most important thing in the discussion. The important thing is to get beyond the rules and look at the relationships to which God calls us, and in this case, into which God calls men and women in marriage, to see that from the beginning God's intention for those relationships was love and unity. I believe Jesus' words are meant to lift the discussion out of a debate about rules, and bring it to a level where human beings reflect on God's creative intention of love. It's the human tendency to forget that creative intention, to forget that rules are not the most important thing. Too often, rule -making religious folks are the ones whose forgetfulness hurts the most.