Let's see if I can try to define civil marriage.
Whilst it shouldn't be a requirement, it is generally for the people involved used to declare their love for one another. Then we move onto why they would have it. The definition is never going to be as important for why we need it. And it's summed up by "legal rights", I once skimmed through a 100 page document on the rights differences between gay and strait couples. You see that its mostly financial. There are worse rights for house buying, wills, loaning, pretty much everything. Kids is the main non financial one. It is much harder to adopt when gay, I would say this applies to non-married parents as well, in that there are also going to be problems like these along a child's life, if people want that document I may be able to find it somewhere, someone emailed me a link a couple of years back.
Now, after some thinking, I'll see if I can come up with a clear definition, try: "the official recognition of a relationship."