Originally Posted by Ron Conte
The marriages of the children of Adam and Eve, to one another (brother-sister) were not intrinsically evil. No marriage in the collateral line is intrinsically evil. A marriage in the collateral line can be invalid or a grave sin, but not due to the fact that it is collateral.
I think I got it now.
Marriage in direct line (e.i. parent or grandparent - child) = intrinsically evil and always immoral.
Marriage in collateral line = not intrinsically evil, but due to the circumstances in the world today (big population - people can easily marry a non close relative person), it can be invalid and a grave sin.