Characters in stories frequently pick up on things faster than one might think realistic.
Though this can be a result of terribly bad writing that a professor would admonish severely, it can also be the lesser evil when given the choice between regurgitating information in excessive detail that the audience is already well aware of, or getting to the next point in a story.
This is quite similar to other things in stories taking less time than they would in real life, such as accelerating how quickly a court case is handled relative to how long it would take in reality for the sake of a story's pacing.
None of the above, however, is applicable at all to Elliot's immediate understanding and acceptance of Ashley's interest in, and writing of, stories with these themes.
The actual explanation for this can be given in a single word: