Infidelity is not generally referred to as abuse, or even thought of as such.  But I would propose that it is.

“According to the Gale Encyclopedia of Medicine, the definition of abuse is the following: Abuse is defined as any action that intentionally harms or injures another person.  In short, someone who purposefully harms another in any way is committing abuse.”  Source: https://www.healthyplace.com/abuse/abuse-information/what-is-abuse-abuse-definition  Retrieved 8/10/19

Someone who is being unfaithful to the partner with whom he/she is in the type of relationship in which both individuals have agreed that they are committed to loving each other and practicing monogamy, knows what they are doing and knows the likely effect on the other person.  I.e. The other person is harmed by the behavior, and the behavior is done on purpose.

The harm.  I would argue that even when the person who is being cheated on is unaware of the infidelity, he/she is being harmed.  I’m not saying that the two people should stay together as a couple, but I am saying that cheating on a partner is not a healthy or emotionally mature way of dealing with a relationship that one isn’t satisfied with for any reason.  And furthermore, it is cruel.

When the person being cheated on becomes aware of the infidelity, the impact on his/her health can be huge.  And often is.  The emotional pain felt in response to finding out that the one with whom you believed you were in a monogamous and loving relationship, has been unfaithful, could be described as a stab to one’s emotional heart.  A stab to one’s soul.  And it’s a shock.  A horrible shock.  Even if previously suspected.

And unfortunately, it isn’t only a one-off, momentary sort of pain.  On the contrary, following the initial stab and shock, comes the rolling, continuing pain.  Immense emotional pain.  Potentially, clinical depression.  The deep pain and the thoughts that go through the mind of the wounded party can be consuming, unless one is in a denial phase, (‘Denial’ being one of the usual phases associated with grief and loss, that a grieving person is expected to experience).  Probably the intensity of the pain will lessen over time, but the pain can last for years and even remain forever.  One has been hurt to one’s core.  Betrayed, harmed, abused.  Traumatized.

“Psychological abuse, often called emotional abuse, is a form of abuse, characterized by a person subjecting or exposing another person to behavior that may result in psychological trauma, including anxiety, chronic depression, or post-traumatic stress diorder.”  Source: https://en.m.wikipedia.org/wiki/Psychological_abuse  Retrieved 8/10/19

“Adultery is no longer a crime in any European country. … As of 2019, adultery remains a criminal offense in 19 states in the USA.”  Source: https://en.m.wikipedia.org/wiki/Adultery#United_States  Retrieved 8/10/19.  In New York, adultery is “technically still a crime …, though it is never prosecuted.  It is listed under New York State penal law as a Class B misdemeanor.”  Source: https://brooklyneagle.com/articles/2019/04/30/adultery-is-technically-still-illegal-in-new-york-in-one-brooklyn-trial-the-defense-is-trying-to-use-it-to-their-advantage/  Retrieved 8/10/19.