The believer should not pursue a divorce from an unbeliever, but if the unbeliever wants out of the marriage, the believer should not contest this, and the believer would then be free to remarry.
The two grounds for remarriage, then, would be a divorce that involved sexual immorality, and a divorce initiated by an unbeliever.
It gave them the freedom to re-marry, without the guilt of committing adultery.
Jesus was even more restrictive on divorce than the most conservative Jews.
I think all Christians would agree that God's ideal for marriage is "one man and one woman, living together in marriage until parted by death." We see this emphasized by our Lord in Matthew 19:3-12.
As strict as the Pharisees were, they believed that divorce was permissible.
While Jesus looked upon divorce as a "necessary evil" for those who live in a fallen world, it is never looked upon as good.
Divorce, Jesus said, was never commanded, but only permitted.