The definition of sexual abuse in our culture can sometimes be a bit narrow. Obviously involuntary sexual activity or activity that occurs before a person is capable of giving consent is abusive (it also results in the individual picking up the carried lust of their offender). However there are numerous forms of sexual abuse that are much more subtle. For example, exposing a child to sexually explicit material (i.e. pornography) and/or circumstances (i.e. parents/partners having sex with the door open or in the same room as a child) both create similar patterns of compensatory responses as the "traditional" examples of sexual abuse.
Another common example is the "daddy's little girl" or "mommy's little man" situation, in which the child meets all of the spousal needs of the parent except the physical sexual needs. They may serve as confidant, be falsely-empowered, be asked to join in on major financial choices, etc. Also common is for a young woman to suddenly garner the attention of her father as she starts to develop sexually.
These are all "covert" examples of sexual abuse. Again, the reason we refer to them as sexual abuse isn't simply to be Victorian or prudish - the adult who experienced these covert behaviors as a child will develop the same set of compensatory mechanisms (often promiscuity, sexual preoccupation, and/or periods of sexual anorexia - all related to carried lust) as folks who went through the overt abuse.