I'm not sure why the disagreement with civil unions is so strong.
Of course, if they were implemented fairly those espousing them now would go berserk. :)
First all references to marriage would have to be removed from all laws. Marriage would be a personal custom in some subcultures with no mention or support from government. Religious positions would not automatically make one able to preside over a CU (civil union). In fact, anyone who is allowed to issue a CU would have to be licensed. Perhaps Notaries would be able to do this automatically without further licensing.
In other words, the existing definition of marriage would be replaced by CU and extended to others than marriage is now.
Of course, individual subcultures would be able to marry whomever they wanted. Marriage would have no special meaning other than that given it by the subcultures. All existing legal marriages would be grandfathered in and relabeled as CUs.
This would be totally fair. Everyone would get all the same rights in law. All subcultures that have some attachment to the word marriage would be able to continue to offer ceremonies and call it marriage it they liked. They could set whatever criteria that they wanted to allow or disallow access to their local ceremony.
Others could, of course, use the word marriage to mean participation in their own local ceremony.
Those that worry about the "destruction" of marriage might want to reconsider the use of CUs in place of marriage.