Griffin wrote:why is it not appropriate for the legislature to assign providence of what was originally a religious institution back to religious organizations
Marriage very well could be argued to be a religious institution. But civil unions? A justice of the peace can do all that without reference to god (or God) and the like. That is a legal matter for reasons that Dana specified above. The two happen to overlap for the most part.
I would argue that sexual reproduction is not a religious institution. Just ask your dog. And check out what the trees, birds, and bees are doing every spring. For we humans, it is an unalienable right of (wo)man.
Whether or not you want to call "it" separation of church and state (and I think you have this wrong, but I am not a lawyer or judge...) you cannot deny someone the civil right to union on the grounds that they do not recognize the religious institution. THAT is a violation of the U.S. Constitution.
How we define that civil union is another matter altogether.
For the record... I think religions should be allowed to keep "marriage" as "marriage" and however they want to define it. But if you want the civil union and all legal rights that come with it, that is a matter of the state. As such, I am against gays and lesbians demanding state recognition for "marriage", but all for the civil union business.
I see no reason to spare them the agony...