According to Wikipedia (and you can take it for what it's worth), the one stated difference in civil partnerships in the U.K is that a member of a civil partnership is not allowed to use a courtesy peerage title which derives from his/her partner. A silly distinction, I'll admit, but a distinction nonetheless.
We humans have a propensity to find someone to whom we can feel superior. Using a different term for the union between two persons of the same sex seems to feed right into that need.
My wife's and my best friends are a lesbian couple of 36 years. I'd love to see them to be able to formalize their relationship here in Michigan, U.S.A. in whatever form. Probably won't happen in their lifetimes or ours. In an interesting side note, one of these two women inherited her family's farm in Michigan as well as a lakeside cottage in Canada. In Canada, she was able to go to a local office and create joint ownership of the cottage with her partner by filling out a form and paying a minimal fee. In Michigan, she was told the only way she could create joint ownership of the farm was to "sell" it to the two of them, thus incurring a sales tax of 6% of its market value. Nice kick in the teeth, eh?