Most people nowadays have the ability to retire in private, away from children to have sex. So to knowingly do it in front of the child would be considered abusive.
Now, there are other aspects. Certainly, people have been surprised in the act by their children for a variety of reasons. The child gets up in the middle of the night and goes into the parent’s room, for example. Or the parents think they are doing it in a location the child may not wander in but find out too late they did. In those accidental cases, one could hardly argue that this was intentional child sexual abuse.