What is the difference between a promise and a wedding vow?

I am a christian, and I am having issues seeing the difference between promising to God that you and your loved one will stay together forever, and the marriage ceremony to begin with. My boyfriend and I intend on staying together forever, and we have made that promise to each other and to God. We have told friends as witnesses, and we both know that we will never separate unless one of us cheats. The religious ceremony that is known as a wedding, basically consists of the same thing, which is the requirement in order to make love to your partner. I am failing to see the difference between the two. Maybe I'm just young and naive, but if two people promise to be together for ever, and mean it with every fiber in their being, what is the real difference, aside from the legal papers and the priest.