What is supernatural about the existence of human rights?
There is no scientific explanation for why humans have intrinsic rights. This is the definition of "supernatural".
There is a scientific explanation for why humans believe that humans have intrinsic rights. What those rights are, and the fact that they're considered to be inviolable (which they obviously aren't) is a question of ethics, cultural identity, cultural history, etc, not supernaturalism. People of the full range of religions and superstitions may believe in human rights, despite disagreeing fervently, even violently on the supernatural.
You believe that humans have intrinsic rights...
I do? That's news to me. Where did I say that?
Believing in human rights is a supernatural claim.
It's a metaphysical position statement on preferred values, viz, that these rights should never be infringed. Not the same as a supernatural claim.
Metaphysics is neither religion nor supernatural: http://en.wikipedia.org/wiki/Metaphysics
Metaphysics is a traditional branch of philosophy concerned with explaining the fundamental nature of being and the world that encompasses it, although the term is not easily defined. Traditionally, metaphysics attempts to answer two basic questions in the broadest possible terms:
What is ultimately there?
What is it like?
A person who studies metaphysics is called a metaphysicist or a metaphysician. The metaphysician attempts to clarify the fundamental notions by which people understand the world, e.g., existence, objects and their properties, space and time, cause and effect, and possibility. A central branch of metaphysics is ontology, the investigation into the basic categories of being and how they relate to each other. Another central branch of metaphysics is cosmology, the study of the origin, fundamental structure, nature, and dynamics of the universe. Some include Epistemology as another central focus of metaphysics, but this can be questioned.