Are we an incestuous race? The children of Adam and Eve were married among themselves. – right.
No doubt the children of Adam and Eve married between themselves. Earlier on there was no prohibition in this matter simply because, it seems, that there was no other way out. Later this prohibition came into affect. This incestuous relationship, if you prefer to call it by this name, was rendered weak in a particular way: The progeny of Adam and Eve was mostly born in the form of twins -- a boy and a girl. These twins were born after many years in between them. The boy of one twin pair married the girl of another twin pair.
In this way, a weakness was incorporated in the relationship. Later on, of course, this relationship was completely prohibited.