We become easy prey to Satan's influence by sowing seeds of disharmony in the Church. This is self-destructive. We are then individually easy targets for Satan. A sad but oft repeated tale. Forgiveness is the cornerstone for a stable marriage.
Forgiveness is love in action.
Without it, the marriage degenerates into a war zone. At the root of many divorce proceedings are unforgiving hearts. Hurts piled up over the years without forgiveness will make fertile ground for deep resentment and bitterness to flourish. Both parties will be easy prey to Satan's manifold devices and temptations. What a heavy price to pay for unforgiveness.