Midrash sur Les Nombres 5:20

(Bamidbar 5:20) "And you, if you have gone astray": This tells me only of the regular mode. Whence do I derive (that the same applies for) the irregular mode (i.e., anal intercourse)? From "and if you have become unclean." "and a man has put his lying in you": to include (in these strictures) one who is impotent, (where there is only "lying," but no seed.) "aside from your husband": to include the wife of one who is impotent. He stipulates all (contingencies) with her.