The distinction between ‘civil union’ and ‘marriage’ is only culturally attenuated linguistics.
To the extent that public law has anything to do with the relationship between any two individuals is has to be non-discriminatory – see Standards of LIFE freedom foundation. So no constitution has any right to try to define a difference between the types of unions between people, or the names used to describe those unions. People are free to call anything whatever they like, but that has no place in law or the constitution.
Societies might do well to start refraining from giving any legal significance to the relationship between any two people. There would be no tax significance once LIFE taxation is in place. Perhaps individuals could append hospital visitation privileges and living will attorney rights to their healthcare records, and if they had not done that, then in an emergency the doctors could accept the decisions of anyone who shares the same address?
Keeping public law out of private spaces is going to be a process of breaking a lot of old habits! And we all know about old habits…