tight fit
The main reason is the anatomy of the genitals. At the base of the penis there is an area called the bulb, which receives a lot of blood and increases in volume when the animal is excited. And females have a fibromuscular ring, with a reduced diameter, at the entrance to the vagina.
until the last drop
When the bulb grows, the ring prevents the penis from exiting the birth canal. Thus, they are “stuck” during and, mainly, after intercourse. There is an evolutionary reason for the system: it ensures that the semen reaches the vagina, increasing the chance of fertilization.
Calm down, what’s up
The bulb decreases only when the male’s erection ends, which can last from 15 minutes to an hour, depending on the size of the dog. Do not try to separate them by force! A sudden attempt can fracture a bone in the penis, seriously injuring the little bug.
Throwing water is also not recommended: scared, the animal itself can force the exit and get injured.
Source Rafael Costa Jorge, general coordinator of the Pompéia Veterinary Hospital