You are...you can't remember, so lets just go with Anon. You are Anon, and you have arrived upon Equestria as the result of a magical spell cast by Twilight Sparkle...your new roomate.
You are Anon, you are fit, you are the only human in equestria, and Rainbow is your best friend. For one reason or another, mares find you desireable, unnervingly so. This is your story.