Sweet, innocent, kind Fluttershy. The very image of chastity. Gentle and pure in every way, no pony would ever have anything bad to say about her.

It is a well maintained image. Different in nearly every way from the true Fluttershy. It is an image she goes through great lengths to maintain. However, she has urges will not be denied.
They just take a bit of effort to satisfy, but with desires like these, Fluttershy's more than willing to put forth the effort. After all, it only makes the reward all the sweeter.