D.C. code is online, and some (take it from me) thorough searching could find no such law.
It did turn up some good material from the city’s obscenity statute, governing what can and can’t be produced, sold, marketed, or shown to minors. Its author started off with his tie a little loosened, but picks up steam:
(B) The term “nudity” includes the showing of the human male or female genitals, pubic area or buttocks with less than a full opaque covering, or the showing of the female breast with less than a full opaque covering of any portion thereof below the top of the nipple, or the depiction of covered male genitals in a discernibly turgid state.
(C) The term “sexual conduct” includes acts of sodomy, masturbation, homosexuality, sexual intercourse, or physical contact with a person’s clothed or unclothed genitals, pubic area, buttocks, or, if such person be a female, breast.
(D) The term “sexual excitement” includes the condition of human male or female genitals when in a state of sexual stimulation or arousal.
(E) The term “sado-masochistic abuse” includes flagellation or torture by or upon a person clad in undergarments or a mask or bizarre costume, or the condition of being fettered, bound, or otherwise physically restrained on the part of one so clothed.