If a husband or wife is committing abuse, they have already broken the vow to ''love, honor, and cherish." Since the vow is already broken, you have not broken it if you leave.
Similarly, if the Spouse is cheating, they have broken their vow. The betrayed party is not breaking the vow by leaving, since it is already broken.
If both parties agree that they are incompatible and agree to part, no vows are broken.