i dont think fat is unattractive just because of the way people look from being overweight, i think it's unattractive because what it says about the person.. either they have no self-control with eating, no discipline to keep themselves in shape, they dont care about their health, or they're ignorant enough not to know....
at the same time, it's just a matter of preference.
i could see being disgusted with sex if it was a really bad experience, or if you find out the person you had sex with wasnt who they claimed to be (ie a transexual, a pathological liar)..
you at least consented to have sex with this girl knowing that you thought she was "fat" before the act occured. you were obviously attracted to her enough to have sex with her. she obviously turned you on somehow... i think you're just upset cuz you're attracted to someone who doesn't fit your (or other peoples) typical standard of beauty. don't be scurred.