I have a notion that this bit (especially the earlier version where Draco admits to not having erections yet) was the result of story constraints.

After somewhat about the Malfoys being pretty cool in ways which are important from the point of view of the story, it was necessary to shockingly remind the reader that they're also morally deficient and seriously bad news. At the same time, it was also a good idea to make this a hypothetical threat-- the story line isn't now (and perhaps not ever) about Hermione being in danger.