Sometimes ending the relationship will harm third parties, but the core relationship is harmful to one or both of the people in it. In a case like that cheating may be the least bad option.
Primarily children. Consider a situation where one member of a married couple is unwilling or unable to engage in sexual contact but is otherwise a good partner and coparent. Divorce is traumatic for children and has a significant negative economic impact, life with single parents is also difficult. So the parent who still wants sex seeks it elsewhere. They are fulfilling an important aspect of their life, and preserving a healthy home life for their children and partner.
Sometimes ending the relationship will harm third parties, but the core relationship is harmful to one or both of the people in it. In a case like that cheating may be the least bad option.
What third parties do you mean? How do the effects on them in either case compare to the effects on one’s partner?
Primarily children. Consider a situation where one member of a married couple is unwilling or unable to engage in sexual contact but is otherwise a good partner and coparent. Divorce is traumatic for children and has a significant negative economic impact, life with single parents is also difficult. So the parent who still wants sex seeks it elsewhere. They are fulfilling an important aspect of their life, and preserving a healthy home life for their children and partner.