Marriage has traditionally been a religious AND a civil union. A religion grants you their blessing and the State grants you various benefits. In my mind the two should be completely separate.
Marriage would be a religious designation, like a confirmation or bar mitzvah, but it would provide not civil benefits, like special tax status. The State would have no authority to compel or forbid any religion from recognizing a marriage.
A civil union would be akin to a contract between adults capable of making their own decisions. The State has no need to judge those decisions, it would only grant them rights for taxes, property, inheritance, etc. A civil union could be between any mix of genders and any number of participants.
You want to get married, find a Church that will marry you. You want civil union benefits, go to city hall.