The US court system is already clogged, this simply keeps one more "brand" of lawsuit out of the courts...Gay Divorce.
Who gets the French Papion, who gets the chiffon curtains...blah...it just gets rid of all that unneccesary bickering and fighting.
::grin::
badpenguin
For all intents and purposes, "Civil Union" and "Marriage" are interchangeable use - for the government, at least. Essentially these two terms describe a contract, giving both parties certain rights and obligations to the other - just like any contract.
I am guessing that in your parent's case, they have the civil unions because they are on second marriages and the church that they belong to does not recognize "second marriages" as valid. The nature of their "contract" has not changed because of what it is called. They have the same rights under a "civil union" as they do under a "church marriage".
The brouhaha around gay marriage clouds this issue, a lot.
At its root a marriage is a legally binding contract, no matter what you call it, if the state has sanctioned it (if you bought a marriage license, then the state has sanctioned it), then it commits both parties to certain duties. Now, any adult can enter into pretty much any contract they choose to with whoever they choose to do it with. The flap around gay marriage is that the state wants to define a certain type of contract as only available to certain types of people. I'm no Constitutional Scholar, and I'm no fan of governements, but this "redefinition" of what a contract is seems, on the face of it, to be beyond the scope of what our government can do (which is why it is all about "Constitutional Amendments", it is the only way that the governement can get the power to legislate against it).
So, to answer your question, no matter what happens, the civil unions that your parents have entered into will be valid (assuming that your parents are in hetrosexual relationships). Everything else about this issue is just another front in the Culture War, and just another attempt by the government to legislate morality.