Great comments from John in Kansas and almost everybody else. If two people of any persuasion whatever want to be joined in a civil union for legal reasons, there is no reason why the State should not issue the permission slip known as a license. (Sure, subject to sensible qualifications of age of consent, not already civilly unioned to somebody else, etc.)
If those same people choose to be joined in the civil union in the church, synagog, mosque, etc of their choice, for reasons of sacrement or tradition, then go for it and much luck and joy to them. You can call it a marriage or whatever language works in your tradition. And if your tradition does not recognize your choice of partner, maybe this is not a real great tradition to be part of anyway!
If you choose to have your civil union celebrated by the mayor at the local firehouse (been there), thats great too.
I still call my former significant other my spouse, and she calls me, well, lots of things, but I digress!
The point: It's a civil union in all cases. To argue about if it is a marriage or not, is one of those things which hey, if it important to you than OK, but the word marriage has no bearing on the legal status of the couple.
Mark