Paul says that the saved spouse not to fight or contest the departure. It is obvious that Paul is describing a case where the saved spouse is not the blame. The unsaved spouse for reasons not given will not live with the saved spouse and chooses to leave. Paul says let him leave. There is not to be war but peace.
Marriage is considered as a bond.
We often speak of the bonds of matrimony. Marriage in God's sight is a bond between a husband and a wife. Whenever that bond is broken in an allowed fashion, a person is free to remarry. Any other breaking of the marriage bond is denied as we consider earlier. But Paul tells us that in the case of an unbeliever leaving a believer that they are no longer under bondage.