Yes, infidelity is a form of abuse, because people in a relationship play mind games with each other when they cheat. Anything besides total commitment to one's partner is abuse. Having an affair with someone else while married is extremely emotionally abusive to the other person. It is a terrible thing to do to a spouse.
Infidelity in a relationship has the ability to not only tear the union of two people apart, but it usually tears apart at least one person from the original union. Through the emotional burden of being heart broken and depressed, to the wondering whether or not the infidelity started because of something they did, it always affects the victim negatively.