How are you not married legally? Is your "spouse" still married or can't get out of his current marriage? Outside of that, there shouldn't be many reasons why you two can't tie the knot.
Okay, for argument's sake, let's say you and your spouse are living together and having kids (why not - one commitment is as good as another). You are their mother and he is their father. You two have taken a vow of fidelity without a marriage, church, or justice of the peace making you man and wife - depending on what state you're in, you may already have a common-in-law marriage.
In reality, marriage is an agreement between to people to be true to one another for the rest of their lives. God does not give a marriage and the law doesn't either. The priest, reverend, justice, or judge is there to witness it, along with the congregation. Two people marry each other.
If you both agree to that without any kind of legal officiant, you're married - Mazel tov. For all intents and purposes, you're a couple. While I would got short of introducing yourself as Mrs. Marriedperson for legal reasons, unless you're looking for a divorce your unofficial civil union should be good until one of you dies.
But then again, I'm not God to most people. If it's good enough for him, who am I to argue?