It has to do with what you find sexually/aesthetically attractive versus what you would actually do the do with.  Real, human-sized asexuals do exist, and they may or may not find whatever sort of people hot, so that's when you'll see people use more than one term. Fat "asexuals" though, hah, laughable. You don't get to say "I never wanted any anyway!" without people going "You were never gonna get any.