To frame it in civil rights terms (IMHO):

Civil Union = Separate but Equal
Marriage = Equal Rights

That's my take. As much as Separate but Equal was wrong, denying the GLBT portion of the popluation marriage rights is wrong. It's a fairness issue. I don't see where history or religion should have any bearing. I know there's a lot of people who don't agree. Racists don't think blacks and whites should marry either. It doesn't make them any more right by saying that 'historically blacks and whites haven't married'.

I'm not ducking.

-Zeke