Shoujo Renai Heni

"A world in which everyone is born female and some become male upon maturity (a bit like in "Simoun"). Momoka and Arisu are very close friends, and Momoka wants to become a man and marry Arisu . . . only when they end up in danger from a rapist, it is Arisu who suddenly turns male! Arisu's transformation seems to be unstable though, as "he" returns to female again when the adrenaline rush fails."

(Source: MU)