When one of the partners is engaging in repeated infidelities.
In abuse cases, the partners should separate, and the abuser should be highly regulated as to their behavior in order to get access to their children. The abuser must agree to a strict conduct agreement, and if they violate it one time, we're leaving back to the safe house. It must be strictly enforced, and not give the abuser what they want. In time they will learn that they will not gain anything for their abusive behavior, which may encourage them to change their behavior.
Letting them out of the marriage doesn't work. The abuser will just find someone else to abuse. Also, the victim should not be running off into a new relationship without healing from their trauma and learning from their experience. So they need to do the "separate and regulate" approach, and get supporters to back them up on it. Preferably supporters who have weapons and aren't afraid to use them, but you get what I mean.