A healthy relationship can see branching out as an enjoyable thing, if done correctly. Twilight and Fluttershy have been together long enough that they certainly can at least begin trying; after all, why shouldn’t they attempt something new? With one another is safest.

Twilight just didn't expect her girlfriend to want to be dominated a little. And seeing as she's no controlling mastermind, what is she to do? It's not like there's anyone to guide her-

Or perhaps there was.