With marriage you can't separate the legal from the intimate, or the religious from the personal that easily. I have friends who have married their partners, and friends who haven't. They have a collection of stories and a myriad of reasons. Humans are growing and loving and the old rules and customs are having a hard time keeping up. Then there's Valentine's day. What other day do we celebrate our partnerships without the rules. The origins may be sketchy, and some early celebrations included being smacked with goat hide, but what's wrong with a day when we are given a little nudge to celebrate our intimate companions. Sure it's commercialized, but try to tell me marriage isn't.
As my relationship with my partner evolved we made commitments that would have been there had we married, or not. I debated how I felt about marriage for some time while being fully committed to our partnership. It turns out we did get married and we chose this day to do it because our life together will be what we make it - it will include the lightheartedness of candy hearts and the weight of the many hopeful declarations before us.
Happy Valentine's day!