Remaining Together without Fail
While life is not expected to be perfect, many people have the belief their partner should be perfect in all ways. This is unrealistic, and no one can be held responsible for having bad days. A partner who expects this type of behavior will soon be ready to move on to another relationship, or they may use it as a justification for cheating.
A serious long term relationship is one where a person makes a commitment to stand by their partner in almost all circumstances, and a person who chooses to go outside the commitment for physical pleasure is making the statement that the relationship does not matter. Cheating on a partner is one way to let them know they are not considered worthy of a commitment, and the person who cheats may be unable to understand the message they are sending. When caught, they generally have their excuses ready so they can blame it on the other person, and they are shocked when that person ends the relationship permanently.