Here we are discussing incest as a question of whom one may marry (not incestuous child abuse).
Incest (of blood relations) is divided into two types:
1) in the collateral line -- this applies to:
siblings (first degree of the collateral line)
first cousins (second degree)
second cousins (third degree)
third cousins (fourth degree)
2) in the direct line -- this applies to:
parent - child
grandparent - child
or any similar direct lineage relationship
Marriage between persons who are related in the collateral line in the first and second degrees (siblings and first cousins) would make a marriage invalid. For third and fourth degree relationships (second and third cousins) a dispensation is needed, or the marriage is invalid.
Beyond the fourth degree, no dispensation is needed. For we are all members of the same human family. You essentially cannot marry someone who is entirely unrelated to you, because then they would not be human.
The Jews in ancient times preferred to marry other Jews, and they often preferred to marry within their own tribe.
So marriage is not intrinsically evil with consanguinity in the collateral line (blood relations, but not direct), because it admits of degrees in which some degrees are not immoral. And the Church could not give a dispensation for second and third cousins if the marriage were intrinsically evil (it would also not be valid as a marriage if it were intrinsically evil).
But a dispensation is never given for consanguinity in the direct line, i.e when one person is descended from the other (parent/child; or grandparent child; etc). Nor was such a union necessary even in the earliest days of the human race. For we are NOT all related in the direct line.
So, in my opinion, incest in the direct line is intrinsically evil and always gravely immoral. But marriage in the collateral line is not intrinsically evil; it depends for its morality on circumstances, such as the degree, and whether there is a compelling reason (and in modern times a dispensation).
Could the children of Adam and Eve marry and bear children without sinning gravely? Yes, since they were only related in the collateral line. Moreover, we can conclude that God granted a dispensation for the close relationships in the collateral line in that case, due to the necessity of the survival of the species. But after very little time had passed, it would no longer be necessary, and therefore no longer be permissible.
Roman Catholic theologian