I believe in marriage. To be fair, though, I think I'm answering a different question than the one you're asking. To me, marriage is a state of mind and I think what you are really asking is, "Do you believe that a public ceremony, and the signing of various documents, is necessary for marriage to take place?" And my answer to that is "No".
That may be oversimplified, and don't get me wrong: I'm not putting down the public ceremony, or the signing of documents, or any of the other elements of that event. But it is an event.
The marriage is a relationship, which, to my mind, may be aided by the event, but does not require it in order to exist. The commitment that makes a marriage does not start with the wedding. It starts before that, sometimes a long time before that, and it grows, as long as the marriage is alive. When the commitment stops growing, the marriage starts dying, and all the public ceremonies and signed documents in the world aren't going to change that.
There are lots of good reasons for having the event: social, religious, etc. There are also good reasons for not having it. I believe it is 1000% up to the couple to decide what is important to them. Is it important to have an identifiable point in their lives when they stopped and spoke their commitment to each other in a public way? Then do it. Is it important to them to sign legal documents that say they have done this? Then do it. Is it important to them to have a celebration of this decision that includes whoever is close to them? Then do it. Those are all good reasons, and there are tons more.
For some, though, good as those reasons may be, they are not sufficient to counter some other consideration, specific to their situation. I'm not going to go into detail here, because I've already taken up enough space, but this does apply to Rob and me. We have our own reasons for not having the event, but no regrets. It makes defining an anniversary difficult, but if that's our biggest problem, I think I can handle it.
So, for the sake of argument, let's take as an anniversary the date we actually began living together -- when pressed, that's what we do. After 15 years, people still get a kick out of us "middle-aged teenagers". And you'll just have to take my word for the commitment.
I guess for me what it boils down to is this: those friends of yours, who "aren't married"? they are -- they just didn't have a wedding. And what you're really contemplating is the all too frequent demonstration these days that you can have a wedding, but not necessarily have a marriage. Well, it works the other way, too: you can have a marriage, without having had a wedding. Oh, and do I know people with happy marriages? Yes. A good many of them -- many of whom DID have the event. Don't give up on it.