Why does a Male dog's Penis get stuck inside the Female, just after they are done with sex

I have noticed many times that when a male dog is done having sex with a female, he needs to struggle real hard to get his organ out of the female. They could continue this struggle for more than 30 minutes.

 

Because, unlike with humans, they are basically not 'done' with sex. It's still going on. Although it's not necessary for a male to tie with his bitches for there to be a successful mating, this is what most breeders like to see - otherwise it's often necessary to hold the male in for a reasonable length of time, during what's called a slip tie. One of my successful studs (he had a 99.9% strike rate) never tied with his bitches. Dogs who struggle must always be restrained, as serious damage can be done to both parties, if they try to separate, or somebody tries to separate them, during a tie.

He shouldn't need to struggle, the penis will gradually go down, over time, so they can come apart naturally. Ties can take anything from 20 minutes to up to an hour.