not a metric. not the spirit of a relationship. its a specific action a couple takes to fromalize the idea that they are sticking together. no one has to do it (ideally. check with your society to verify) necessarily. Now you get into the legal aspects which is mentioned a bit in the post. In the us you can add your spouse to your insurance and children but generally not others. Its one reason I kinda hated that they passed gay marriage as many states and businesses and insurers where allowing for broader ability to add people you live with. It eases inheritance stuff and there are some others for the us anyway. other countries may have other things. I mean my wife and I decided to not have kids. If we did we would not have without the legal cert around it.