I believe that if we are in a committed relationship, then we have already made a promise to be faithful to that person. The only problem with not doing it legally, is the "laws of the land/country" we live in, can make it difficult for marital benefits, etc., if you do not comply. Also, we are still very steeped in the traditions of our culture, which recognize the moral issues of marriage. There is also the issue of children and how they are generally regarded when their mother and father are not married. There is still a stigma in society about how matters like this are viewed.
Setting up a household with someone is different than the dating thing. Sometimes we have to explore the sexual end of things, as well as really getting to know that person, before we know that the person is that special someone. We see other failed marriages and we don't want to rush into that decision, and then later on, find out that we have made a mistake.
I like being married, and appreciate the legal aspects of it in these times. But, after coming out of the JW's and leaving a very abusive marriage, I was very afraid of relationships. I did something I said I would never do and that was to live together, as a couple for a year before we got married. At the time, I didn't give a hoot what anyone else thought. It was a private decision made for private reasons. It worked out. We did get married. I think it was a good thing.
I would never say that you are "living in sin". You are living in "life". You are responsible and accountable for your own decisions and choices, and you are free to make them. Be happy.Be wise.