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.