Re: not to sound like a moron...
Gender-buggery or gender-fuck indicates a fic where one of the partners in the pairing (or in this case, threesome) either changes genders OR has genitalia or develops genitalia of the opposite gender OR (in the case of HP and other magical universes) a spell or potion is used that provides one or more of the partners with the necessary genitalia to fuck or be fucked.