Mark 10 includes another version of the events recorded in Matthew 19.
1 Jesus then left that place and went into the region of Judea and across the Jordan. Again crowds of people came to him, and as was his custom, he taught them. 2 Some Pharisees came and tested him by asking, “Is it lawful for a man to divorce his wife?”
3 “What did Moses command you?” he replied.
4 They said, “Moses permitted a man to write a certificate of divorce and send her away.”
5 “It was because your hearts were hard that Moses wrote you this law,” Jesus replied.
6 “But at the beginning of creation God ‘made them male and female.’ 7 ‘For this reason a man will leave his father and mother and be united to his wife, 8 and the two will become one flesh.’ So they are no longer two, but one. 9 Therefore what God has joined together, let man not separate.”
10 When they were in the house again, the disciples asked Jesus about this. 11 He answered, “Anyone who divorces his wife and marries another woman commits adultery against her. 12 And if she divorces her husband and marries another man, she commits adultery.”
One significant distinction between this passage and Matthew 19 is the failure of Mark to include the exception for fornication. Some have concluded that the safe course is to take the more conservative rule — and so not even permit an exception for fornication. But this approach to scripture makes Matthew into an inaccurate reporter. None of the Gospel writers wrote everything that Jesus said — some editing was necessary for obvious reasons.
One reason there’s no exception for fornication in Mark’s account is that fornication wouldn’t have been appropriate as Mark quotes Jesus. In verses 11 and 12 of Mark’s account, Jesus simply declares it to be a sin to divorce in order to remarry. As we’ll explain later, the Greek implies that the divorce and remarriage are concurrent. Fornication is not an exception to this rule as the reason for the divorce is to marry another.
In verse 9, Jesus simply declares that it’s wrong to break the marriage covenant. And, of course, fornication is not an exception to this rule — it’s an example of this rule.
Finally, Matthew was written for Jewish readers, while Mark was written for Gentiles. The exception for fornication is mainly of interest to Jewish readers, because it’s an interpretation of “indecency” in Deuteronomy 24, a point of little interest to Gentiles.
Once we understand Mark’s account, we see why Paul didn’t mention fornication in 1 Corinthians 7. Both Jesus and Paul agree that it is wrong for a spouse to violate the marriage covenant so as to end the marriage. Fornication is just one of many ways that could happen.
 There is considerable but not conclusive evidence that Matthew was originally written in Hebrew or Aramaic and that we have a Greek translation. In any event, the internal evidence of Matthew certainly supports the view that it was intended primarily for a Jewish audience.
 Mark is generally considered to be based on the teachings of Peter as written down by John Mark, very late in Peter’s ministry, when he was preaching largely to Gentiles.