The trouble is that the word "marriage" has religious, social, and legal meanings now - and the different meanings get mixed up. When you get married there are three parts to the process to reflect this:
the religious ceremony: even civil weddings usually have some spiritual/symbolic aspect;
the legal ceremony: usually a hurried affair of signing the papers;
the social ceremony: i.e. the reception - speeches and so on.
Leave out any of them and you don't "feel married", as most of you who are married will no doubt attest. If you miss out two, people tend to really notice... and this is an emotional topic so feelings are important. The point is not just to get hold of some assorted legal benefits and obligations but to convince everyone who is important to you that this relationship is serious - you both really mean it this time!
Steps 1 and 2 have the same name ... we have to use the words "in law" or something to distinguish them. Either one would be a marriage - but you are only married in law if you do step 2.
Step 3 has a different name. It is not normally thought of as part of the marriage itself but if you leave it out the feeling is different: it feels like you don't really mean it. So - everybody does something.
The equality folk want step 2 to be the same contract for all couples. Since this step is, in real life, separate from step 1, you'd think there wouldn't be much fuss.
It would be so much easier if they [steps 1 and 2] had different names.
We could strike "marriage" from the law books and replace it by that different name. That would be logical and neatly gets government out of the marriage business.
There are some problems with this.
It doesn't feel the same - people don't want to get "civil unioned" they want to get "married". Probably just need a better name than "civil union" but why can't you just call yourself "married" if you feel like it?
This is what is actually going on: religious people want to reserve the word "marriage" so that not just anybody can claim to be married. Just like not just anyone can call themselves a lawyer or a doctor. But, instead of being about a professional standing, it is more about branding. People claiming to be married in religion are claiming a status for their relationship in the context of that religion. So it is more like not everyone can claim their touchscreen tablet computer device is an "iPad".
It is about the (market) position of that religion in society, their social identity is under threat, they want to protect it.