I can't say I understand the concept of asexuality, but a highly sexual person myself, but I think I can understand how this would make you feel. I would imagine it's almost the same as feeling undesirable, which I can certainly relate to.
My advice would be to think about your relationship and your attachment to each other. How is everything else between you? If all else is fine and you have that emotionally/spiritual connection, I don't think it should matter in the long run. Remember, he enjoys sex with you; he desires you as a person and appreciates the sexuality you have between you both, it's only because he's not a very sexual person does he have this low sex drive. He IS attracted to you.