Rule 63 is an internet rule that means "For any given male character, there is a female version of that character." This also apply to female characters turning into males. Unlike Rule 34, there are two exceptions to this rule. A: the male character is already so androgynous that a female version would be basically the same, or B: the female version hasn't been drawn yet.
Man: Chun-li is so sexy right about now.
Woman: Bah, here's a Rule 63 Chun-Li without boobs and now with a penis.
Man: Ok that's wrong...Now let me show you Dante with a Vagina and Brest.