Midrash sobre Salmos 89:3

11) "and sees her nakedness and she sees his nakedness": (In order for both to be liable to kareth) they must both transgress wilfully (and not unwittingly). "it is chesed" (connoting "shameful") And lest you say: But Cain married his sister! It is, therefore, written "it is chesed (sometimes connoting lovingkindness) and the world, in its beginning, was created only with lovingkindness, viz. (Tehillim 89:3) "For I have said: The world will be built by chesed."