To play devil's advocate here, why do we need alternatives? You are either married or you are not. A common law marriage is a legally-recognized type of marriage and should be treated as such in the marriage field. Unmarried, partner, and other forms of relationship that are not legally considered marriages would not be reflected in the marriage field but just in the bio. Parentage fields are independent of marriage, so no issue there. Wikitree policy is to base parentage purely on biology, so marriage is irrelevant for lineage.