"Legal" marriage is an historically NEW concept. 500 years ago (and less in lots of places) marriages between the lowly peasants did not happen (and lets be honest now, we are not all related to Lords, Ladies, Royalty and Peerage). There was no written document. There was no where to file it, no bureau of vital statistics. People just got married, period. Sometimes a church ceremony, sometimes not. They lived together and provided for themselves & their families. No fuss, no muss, just get married and get on with the business of providing for your family.
Then the "gentrification" of society happened. The industrial revolution, cottage industries, factories, and everyone starts reading and writing. Voila, the ceremony is codified, the procedures and legitimacy sanctified by "registration" with the state.
Ok, I have over simplified the historical perspective. However, you get the idea. Marriage was not always codified as between a man and a woman.
If a white man married an Indian woman, she was no longer allowed to live on a reserve. She lost her Indian status and her children were not deemed to be Indian. In some states a black man or woman could not "legally" marry a white man or woman.
My point: these ideas of marriage changed over time. It changed to reflect the changes in society. Operative word: changed.
We need to ackowledge that people have a right to marry. Period. Get over ourselves as being the sole arbiter of the definition of "marriage". If you believe that marriage should only be between a man and a woman, then don't marry someone of the same sex as yourself, and leave others alone to do as they wish. No one is going to force you to marry someone of the same sex.
Now in the US, there are over 1,000 federal statutes that will affect the government's and a person's rights and responsibilities, based on the definition of marriage. This fact alone suggests to me that there is a HUGE difference between a civil union and a marriage.
Same sex unions happen. They are NOT illegal any more. Lets recognize the fact that marriage is a right and a responsibility, not an operation of prejudice and bigotry.
Happy Pride :)