Why is the penis, a male organ, "阴茎"?
According to this fengshui expert, the penis and the vagina are both named not because of the gender of the bearer, but via Western medical terminology because they originate from the perineum (会阴). 


  “阴茎”这个称呼不是中国人发明的。它属于从西方医学中翻译过来的外来语，与我们中国的阴阳学说无关。从解剖学角度来讲，耻骨至肛门之前的区域叫会阴区。在这个区域的内在“通道”称作“阴道”，外在如茎之物称作“阴茎”，想一想也是很形象的。而以阴阳学说为基础的中国古典医学并不把男性生殖器叫阴茎，而是称为阳根、阳具或者阳物。