"I personally just don't see why someone should be defined by whether they are married or not!"
Well they're not. But from the point of view of this discussion, rights after a break up, the marriage marks a legally recognisable point when assets where pooled and both parties take certain legal responsibilities towards each other.
"I agree that there should be some sort of safe guarding but there is'nt really a way to do it without forcing people to sign a document of some sort"
OK so couples will have to sign a document, and there will also have to be a corresponding legal process to get the document undone. You couldn't just allow one person to run off and withdraw their consent leaving the other high and dry.
So a legally binding signing process and a legally binding cancellation process.
So, as people have already said, isn't that really pretty much what marriage is?
And why bother with a host of new rules, things for lawyers to charge for and cards for Clinton's to sell when marriage does what is required already and the only benefit of this new thing would be for people to be able to pretend that they're not married and come up with some other term for the same thing.