On further reflection, I think I get what magellan is describing:
Hypothetical civil code:
Section 1. “Marriage” is defined as…
Section 2. “Civil Union” is defined as…
Section 3. The set of privileges and responsibilities associated with “Marriage” (see Section 1) and “Civil Union” (see Section 2) is listed below. Any modifications to this list affect both “Marriage” and “Civil Union”. It is not lawful to create exceptions.
Section 3.1. If one spouse is injured the other can make medical decisions…
Section 3.2. Property inheritance…
Section 3.3. Marital confidence in court proceedings…
Section 3.4. Divorce procedures…
… 1000 or so additional sections reflecting the current state of marital law…
Thus, “one set” of laws, I guess, though I’d think of it as two sets with a 99.9% overlap, as long as “marriage” and “civil union” are defined separately.
Does any state actually keep an omnibus list of marital rights and responsibilities? I was under the impression that references to marriage pop up in various statutes across the entire legal code. Tracking them all down and modifying them to specifically expand them to cover civil unions would be quite the undertaking.