But aside from the name, what exactly is the difference between a marriage and a civil union, at least in the way the state government deals with the rights and responsibilities of the two parties?
There are a lot of great answers to this question, but I'm going to focus on one specific issue that doesn't get mentioned much, except at the federal level.
That issue is transferability.
Specifically, if I have rights in one state, do they transfer elsewhere?
On the federal level, they should-- as a married person in Vermont, my marriage should be accepted at the federal level. Currently, thanks to the Defense of Marriage Act, it's not. That may change-- the courts may overrule it or the federal government may enact new legislation.
But... in the meantime... what about other states?
Not all states recognize our marriage. Most, in fact, do not, but among those that do, everybody understands what marriage is, and the laws are fairly clear, consistent and specific. This means that with our marriage, if we visit New Hampshire, we can understand that no one there has any legal standing to deny us the same rights we have as a married couple in Vermont.
If, however, we had a civil union instead? Well, that doesn't mean the exact same thing in every state that recognizes them. The specific rights enumerated vary from state to state and very few people actually understand what, exactly, they mean. This allows for people in states that recognize civil unions to discriminate out of pure ignorance of the law, creating complications and confusions.
Furthermore, there is no federal equivalent to civil unions at any level. The term is a new term, generated as a compromise to appease that Vermont, as a state, eventually abandoned as unequal and discriminatory, adopting same sex marriage instead. Therefore, should marriage equality be enacted on a national level, civil unions will be in a limbo state, having neither clear meaning nor national equivalent.
I'm not writing this to detract from the other piece, which is excellent, but I felt as though this were a bit of a different discussion, more about policy than about Rhode Island in particular.