I'm not sure about that redhead conclusion....I was/am naturally auburn(hair was bright copper as a kid) and my sex life is not that great, but my love life is fantastic
But, because of my hair color and maybe despite it, I have been pegged "firey" and I can be quite emotional.
I have friends that are natural "carrot-tops" and they aren't any more or less passionate that I can see.
I think sometimes we just start living up to the ideas people have based on our appearance, over time. *shrug* Blondes are the fun ones, but it's the simple brunette you bring home to mother, that sort of thing.