Regading penis Dixson showed that it is not really unique and complex. This argument (and there were some others) was used against the idea of human polygynandry.
But what about vaginal shape? I know it is wider as deeper it goes and cervix is convex shaped, which could give an advantage for storing semen. However, I am actually wondering if other primates have similar vaginal shapes (because it can shed light on the question of mating system). Are there any studies on the topic?