To me, marriage IS a civil union right now. What it used to be isn't very important to me. Sure, the word is the same. But the word "doctor" is also the same as it used to be, that doesn't mean I expect mine to use leeches and enemas for every single symptom.
Things evolve and we keep the same name for them. I'm comfortable with that. Marriage isn't a property transfer now, it's a contract between equals. It's starting to become legal in the western world for same-sex people to enter such a contract specifically because now, there is no inequality, so no reason for two people of the same gender not to do it as well.
If your problem is simply with the name of it, I don't see why you couldn't enter one and call it something else for yourself, but I'm not sure why every single other person on Earth would have to change their terminology as well. Marriages are already civil unions, in my opinion, just civil unions we call by a specific term because it's the one currently understood to mean "life partnership".