Rarity can't bring herself to deny her friends anything and they soon grow incredibly bold with their demands. She can't help but feel dirty after doing some of the things they ask, but somehow allowing her own will to be dominated by her friends also exhilarates her - perhaps even the sense of being "dirty" adds to that heart pounding excitement she feels with each new order.

After all, what more can a pony give to her dear friends than one's entire self, body, spirit, and obedience?

PS - This is an alternate Universe. Based a bit on the Nightmare Rarity story arc in the comics where Rarity is shown as somewhat desperate to keep her friends. This is a world in which her generosity is bent into submissive sexuality or perhaps in which it simply expresses in that way.

PPS - This is pre-Alicorn Twilight. Mostly just because I don't feel like writing Alicorns today.