מדרש על דברים 22:20

(Devarim 22:20) "And if the matter were true — if virginity were not found in the maiden": This tells me only of natural coitus. Whence do I derive the same for unnatural coitus? From "And if the matter were true." "if virginity were not found in the maiden": i.e., if there were no witnesses to refute the witnesses of the husband.