All male mammals have bulbocavernosus (BC) muscles, but in nonrodents they are in a different spinal location than in rats and is known as Onuf's nucleus. Most female mammals retain their BC muscles into adulthood to help constrict vaginal opening, meaning that these muscles are only sexually dimorphic in rats.  

Rutgers University

Rats contain the spinal nucleus of the bulbocavernosus (SNB) is a group of motor neurons that innervate the muscles that control the penis, meaning that while males have about 200 of these cells, females have far less and the muscles die at birth. This follows the organizational hypothesis as androgens given at birth can spare SNB cells and their muscles and castration causes them to die. To the androgen-insensitive the muscles also die, meaning aromatization is not a factor. 

Spinal Cord in Mammals

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

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