Bull is penetrated by an assailant who believes he is penetrating a different dog who is also present. There's also an additional scene earlier in the film where Bull is coersed into penatrating someone. These are both played for laughs.
There is so, so much gross-out, graphic sexual content that if this happens to be a sensitivity for you, this is not a film you should watch. There are continuous very crude jokes and sex scenes.
the movie includes the strange practices of dog shows, and at one point a human grabs a dog's penis for dog show reasons. the movie also includes a scene of a dog humping a human.