They shall die childless - Either the offspring should not be regarded as lawfully theirs, nor be entitled to any hereditary privileges, or they should have no blessing in their children.
Matthew Henry
Concise Bible Commentary
These verses repeat what had been said before, but it was needful there should be line upon line. What praises we owe to God that he has taught the evil of sin, and the sure way of deliverance from it! May we have grace to adorn the doctrine of God our Saviour in all things; may we have no fellowship with unfruitful works of darkness, but reprove them.
SDA Bible Commentary (limited)
Volume 1-7
20. Die childless. This penalty may not seem drastic today; but in ancient times it meant much. To die childless was to have no part in the hope of Israel, to be, practically, outside the covenant.