Just a comment on this - why do you feel the need to be legally married? Some folks do have lots of good reasons why, some do it because they feel it's the "highest form of commitment".
You can't legally marry more than one, but you can enter into all sorts of contractual agreements that give most of the trappings of marriage.
There is a long way between marriage and the person being a secondary that the law can't touch anyone on.
"Listen, or your tongue will make you deaf." - Native American Proverb