I've written about the rough love-life of ducks before, and the Muscovy ducks are no exception. These birdies have gone through an evolutionary race leading to not just this sadistic mating behaviour but also in their penis ability. First of all, the females do prefer to pick out their sexual partner themselfs. They especially like male ducks with an appropriate courtship and attracting plumage. But the males are not innate gentlemen, no no no. If a female is unwilling to accept his invitation, he has no objections on forcing himself on her. And what's his weapon of choice? His penis. It's long (up to 40 cm) and screw-like and useful for raping a female Muscovy duck.
But the beauty of evolution has led to a response in female genitalia, because they have a reason for being choosy - they want to pick out proper males that might carry good traits for their offspring. Now, the female vagina twists in such a way that copulation becomes more difficult for males. The hypothesis for the reason behind this construction is that the females can somehow choose who's sperm she wants to be fertilized by, and if she herself has choosen to mate with a male, her relaxation of the muscles makes the reproduction process easier.