The thing with this is that from the perspective of the law, it doesn't really matter if we call it "marriage" or "civil union" if they're legally equivalent. The point is, from the perspective of the state, your marriage is that little piece of paper that you sign along with the rights and obligations it confers. When I read people who, from the left, argue that we need to destroy marriage as a concept and replace it with a secular civil union system I scratch my head because it's the same thing with a different name. Alternatively, when I see people on the right freaking out because they think marriage equality will destroy whatever religious concept of marriage they have I also scratch my head. The church bit is an add on which has little or nothing to do with the state bit, and of course isn't necessary.
It shouldn't be too hard to understand that there is, effectively, a secular state aspect to what we call marriage - mostly a property contract - and then there's the part that's between you, your mate, and, if you care, your church/religion and they ultimately don't have all that much to do with each other.
