The tendency of fiction to assume that someone who is physically violent will likely also be sexually violent. This is most common in Fan Fiction, but it also pops up quite a bit in professional works.
There are myriad possible reasons for the existence of this trope. Some historical precedent exists in the fact that an overwhelming majority of serial killers raped their victims before (or after) killing them. In fan works, there tends to be an alarming and bizarre combination of Draco in Leather Pants and "Not If They Enjoyed It" Rationalization, especially in Boys' Love genre. Sometimes, however, the reasoning is just the opposite; since rape is frequently viewed as a Fate Worse than Death, if a writer wants to create a totally unsympathetic villain, rape is a very easy way to push any character over the Moral Event Horizon, especially if done with a canon character who would never do such a thing in canon.
Compare Aren't You Going To Ravish Me?, when a character makes this sort of assumption, and Gratuitous Rape, which is a common result of this trope. Contrast Rape Is a Special Kind of Evil (when a villain has standards and specifically averts this trope). Compare/Contrast Insane = Violent, which could lead to this trope by association.