Calling it a "Civil Union", or something like that seems fair enough to me, and that's how it's going ahead in England.

Calling it "Marriage" means a change in the meaning of the word, and I think changes in the meaning of words ought to come from below rather than above.

Presumably terms like "husband and wife" or "man and wife" won't apply. Partner always sounds like Bridge or Golf or Business. For some reason the obvious word "Spouse" never seems too popular, whatever genders are involved.