cultural taboos discourage parents using their offspring as sexual objects, true.
from my perspective, morally, one of the things that makes it prohibitive is that there is not an equality of power. a child cannot give informed consent to such a relationship. the lack of equal power destroys that possibility, and because the child has not gained enough psychological and emotional independence, the child, out of fear of loss of support, can only comply.
there are laws protecting children from such predatory behavior, but unfortunately in cases where children are abused behind the closed doors of the cloistered dysfunctional family system, they, unfortunately, have no other model to compare their lives to, and so they are left to feel as if this is 'normal', [probably not a good word], or perhaps rather that such behavior is sanctioned by the larger society.
i am sure there are others here who can explain why laws have been created to restrict such behavior. i only speak from my own instincts here.
using a child for sex is vile and disgusting behavior, and people who do such things should be punished to the full extent of the law [castration anyone?] for such egregious acts.
just some thoughts,
ps: i am glad to see you asking this question, btw. we don't get a chance too often to address these topics that contain the major 'ick' factor. but we need to, because there are many men who come here who have had this experience at a very early age. i am glad you brought it out into the open for discussion.