Actually i believe incest refers specifically to illegality and in the earliest generations since close family relations were necessary to propagate the species it was not illegal and was thus not incest.
I propose that the main reason you (and all of us) have such a strong negative reaction to close family relations is that we have been conditioned by society to feel this way since it has been illegal for a long time. And of course the ever increasing possibility of having offspring with genetic defects is another factor.
As the genome degenerates further and further from its original state of perfection in the days of Adam and Eve through mutations which further accumulate from generation to generation as by snowball effect (we are said to have 100-300 more mutations than our parents and our children have 100-300 more than us), it became necessary to make illegal the marriages first of the closest family relationships (like brother/sister) and later to also ban cousin marriages.
The speed with which the genome is deteriorating is another strong reason (among many) to believe that the human species is very young (about 6000 years) and to of course believe in creation over the theory of evolution. At the rapid rate our DNA is mutationally declining, the species would likely be extinct long before 100,000-200,000 years. This of course is amplified fantastically as our DNA becomes exponentially more deteriorated when we consider the hypothesis that we are descended from single celled organisms about 3.5 billion years ago. Just one more scientific reason to show that the theory of evolution never happened.