Yeah, I tend not to mind the permanent roles in the context of a BDSM fic...but there's a reason I don't tend to read a lot of BDSM fics. Flexible roles make more sense to me because people do change. And even in a BDSM relationship, I think, what they decided on when they began the relationship is not necessarily what they would want five years later. Ironically, the only time I've seen that issue addressed is when the bottom is reassuring the top, "No, I want you to absolute control over me! I need it!"

I can't read fics that use that trope. Categorizing someone's behavior as Slytherin is one thing, because the wizarding world does seem to be very behind the "your House defines you" mentality; saying "The Slytherin stood up and walked behind Harry, and the Gryffindor found himself surprised at the blond's behavior" is silly.