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I think there are two basic answers to your question. The first, and simple answer is:
The more direct, and also more complex, answer to your question is:
As Matthew 19:18 states,
Jesus is clearly pointing out that divorce is, in fact, legal. It is bad, but it is permissible. To turn it into an iron-clad law is then much like laws concerning the Sabbath - Jesus values the individual over the institution, even an institution as great as marriage.
(Note: Not advocating, just explaining the rationale!)
I think that the biggest issue is the exception in Matthew 5:32:
The Catholic Church takes that phrase to mean, "except in the case of unlawfulness" (which is why there is an annulment process), but many take it as an "out." Regardless of the merit of such an interpretation, it seems that that "escape clause" has been used a bit more than it should be.