... Again, we see that these statements by Jesus are far from legalistic rules. They are statements full of compassion aimed to protect the weak and helpless in society, namely, the vulnerable Jewish wife of Galilee. Of course, God did not want the wife in broader Roman society to throw her husband away either, and so the possibly expanded instruction is equally valid for the audience of Mark as Jesus' instruction was for his audience in Galilee. 
A key difference between Matthew and Mark on the question of divorce is the exception phrase in Matthew: "except for sexual immorality."  The fact that Matthew brings out an exception highlights the fact that Jesus' teaching in Mark 10 was never meant to be exceptionless but to give the general rule. Matthew, which the vast majority of experts believe used Mark as a source, brings out the most significant potential exception to the rule--sexual infidelity.
The fact that Matthew and Mark do not mention other exceptions does not necessarily mean that there were no others. Again, the spirit of Jesus in these instructions was not to set up a rule for its own sake but to show compassion on the weak and chastise the pleasure-seeking husband interested only in his own self-gratification. For example, surely it would violate the very purpose of Jesus' instructions to force an abused wife or husband to remain married to a violent spouse.
Of course, just because a man's wife is unfaithful does not mean that he must divorce her. Matthew suggests that sexual infidelity can be a exception to the general rule not to divorce, but surely the spirit of Jesus could forgive an unfaithful spouse. Surely God would be even more glorified by a couple who, after such a significant crisis, could find reconciliation and healing.
Amid all the legalism that has surrounded the application of biblical teaching on divorce, it ultimately comes down once again to the fundamental principle that God is love and that, for Jesus, the lives of real people trumped rules for their own sake. Jesus did not want spouses to harm each other by throwing the other away in pursuit of selfish pleasure. Today he would not want parents to harm their children by ripping a family apart for selfish reasons.
But children can be harmed by some parents staying together as well. Would Jesus want a spouse to stay in a relationship where the other spouse was molesting a child? Surely the spirit of Jesus would want to save some spouses from harmful situations even if the spouse hasn't crossed the line in sexual infidelity. It is the Pharisees in the gospels who mistake the rules themselves for the principles behind the rules.
Our problem is that we know people make excuses to justify what they want to do. We want to be able to stop them from getting by with it. We want to stop the person who wants to claim an exceptional situation when it is nothing of the sort. We humans are very good of coming up with good-sounding reasons that are not our real reasons.
God is not fooled. This is key. We want to be able to stop the person making excuses. We want to expose the fraud. We don't want to leave the pretenders to God. We want to punish them now. But God says that vengeance is his (Rom. 12:19). No one is getting away with anything...
 If this interpretation of how the oral tradition developed is correct, then we see that God used the tradition at every point to clarify his will on divorce in more than one context.
 The key passages are Mark 10:11-12 and Matthew 5:32 and 19:9.