Oddly enough there are several such cases in my family. None of whom have been together for less than ten years (and a couple over thirty years with their kids in college already), none have ever split up despite never getting that State sanction paper and all have the complete trappings of any other married couple. In most cases, they just never got around to it and after many years, it did not matter any more. When you get right down to it, that little piece of paper is pretty immaterial as to whether or not there is a good family environment. It used to be scandalous many years ago in kind of kneejerk fashion, but few people seem to care any more. All that matters is the quality of the people and the quality of the relationship, and getting married does not change that part in any substantive way.
I'll add that few people knew they weren't married either, as everyone just assumes that couples who have the white picket fence, the kids, etc are married. I've discovered over the years that there are an awful lot of people out there who never got around to getting married, and it appears to be increasing in the younger generations.