I'm pretty sure civil unions and marriage are pretty much the same thing.
In most cases civil unions and marriages grant the same rights and privileges, so in that sense they are the same. But in marriage those rights and privileges are understood to carry from state to state, while in civil unions they are not. So in that sense they are not the same, and it is in that sense that my question arises.
In looking into this question I found this FAQ
, which summarizes it like this:
Even though each state has its own laws around marriage, if someone is married in one state and moves to another, their marriage is legally recognized. For example, Oregon marriage law applies to people 17 and over. In Washington state, the couple must be 18 to wed. However, Washington will recognize the marriage of two 17 year olds from Oregon who move there. This is not the case with Civil Unions. If someone has a Civil Union in Vermont, that union is not recognized in any other state.
On the basic matter of transferability between states, marriage and civil union are not the same. (I don't claim that this site should be used as a legal authority on matters beyond that basic fact.)