Can't really be sure if the bok choy is supposed to mean something specific. All I see is Yuuka hovering over Yukari, tossing salad while a baffled Kogasa looks on.
To tell the truth, most of Ryuuichi's pictures (and his comments about them) contain a joke/parody about that gay porn , Wrestling one or "Cookie☆". He is what they call Inmufag. So his pictures are originally certain scenes from gay porn. Still, his drawing is really fascinating XD
