Marriage is not just a religious thing in the US. It is a legal term, and it affords benefits that civil unions do not. Since civil unions differ from state to state, the benefits that a marriage automatically affords to a couple may not be come with civil union status. This means things like medical decisions, child custody, visitation rights, and a whole ton of tax, property, and insurance issues might not be in a partner's hands in case of death or emergency.
There is a separation of church and state here, so granting gays the right to marry would not make it mandatory for churches to perform those marriages. Churches that wanted to could, and gays would have the right to get married by justices, and everyone else who has the legal right to preside over a legal marriage in the US. Contrary to popular belief, this have absolutely nothing to do with anyone's religious rights.
"It's like arguing with a brick wall, except the brick wall thinks you're an idiot, and thinks it's winning." - Humphreys, that sexy beast.
Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn