You may disagree all you like, however your argument remains flawed- the truth of the matter is that you're [strawman] of exchanging the terms “marriage” for “civil union” doesn't fly, and beyond that you're concept of “Tradition” (or belief, often one that has been handed down from generation to generation) is far from accurate.
It stands on firm ground that, “There is no good reason to prohibit same sex marriage. It has proven to be harmless sociologically” is not an accurate proposition.