Why do female dogs have their private parts stick out?

The structure that is protruding from a female dog's vulva is not her private parts, but rather her clitoris. The clitoris of a female dog is homologous to the clitoris of a human female, and is a primary erogenous zone. It is much larger and more prominent in dogs than in humans, and is located just below the opening of the urethra. During estrus, the female dog's clitoris becomes engorged with blood and protrudes from the vulva, making it more visible and sensitive to stimulation.