I call this simplistic in part because it fails to define what constitutes consent. Generally speaking, abuse is harmful. Sexual abuse is sexual actions done without one's partner's consent. This ranges from feeling someone up without their consent to sexual intercourse. Sexual abuse is often combined with physical abuse, as in violent rape.
Some people believe that all sexual contact with children is automatically sexual abuse.
Sexual contact between two relatives is incest and is a form of sexual abuse in some cases, such as a parent and its child. Consistently telling someone that they are worthless is emotional abuse, for example. Telling someone they are not worthy of love is emotional abuse. Withholding of affection is also emotional abuse. Manipulation of someone so that they do precisely what the manipulator wants is emotional abuse.