Matthew 19 is for national Israel. Jesus was not implementing new marriage laws. Rather, he was expounding on the true spirit of the existing Law.
In
I Corinthians 7, Paul gives the New Testament church clear instructions on marriage. He makes it clear that a Christian should not initiate a divorce, and especially if two Christians seperate, they should reconcile or remain single.
"To the rest", Christians married to unbelievers, he makes it clear that they are not under the same obligations as the first group provided it was the unbeliever abandoning the marriage. I know some will insist that the Christian abandoned by an unbelieving spouse cannot remarry, either, but such a conclusion makes Paul's words seem meaningless.