Consider that the federal government does not recognize civil unions for tax and most other purposes, so any statewide measure that ended marriage entirely for everyone would mean that everyone was unmarried for tax purposes.

Marriage means something that "civil union" does not and never will. As a straight person, I will not accept second-class marriage for myself, or for my friends in same-sex marriages. We need civil marriage equality for everyone.

And for practical political considerations, the chance of getting a federal "civil unions but no civil marriage" law passed is less than the chance of passing a federal marriage equality law. So we need to remain focused on civil marriage, not civil unions.