Adam Clarke on Leviticus 15:24

The common sense of all mankind has led them to avoid the gross impropriety referred to in this verse - and it has been a general opinion, that off-spring obtained in this way has been infected with leprous, scrofulous, and other deeply radicated diseases, from which they and their posterity have been scarcely ever freed. In Lev 20:18, persons guilty of this are condemned to death; here only to a seven days' separation; because, in the former case, Moses speaks of the act when both the man and woman were acquainted with the situation: in the latter, he speaks of a case where the circumstance was not known till afterwards; at least, so it appears these two places should be understood, so as to be reconciled.