How Is Marriage Defined?

Same sex marriage is in the news currently due to cases being heard before the Supreme Court, and that got me to thinking about MY marriage.

We are a traditional male-female couple but we weren't married in a church.  We were both raised Catholic, but neither were active in the Church at the time, and due to her being active duty Army and stationed in Germany, unable to meet the Church's pre-marriage preparation requirements.

Nearly 26 years there was a small ceremony in my parents' living room, officiated by a county Justice of the Peace and witnessed by a select group of family and friends.

We were both of legal age, both standing before the JP of our own free will, and both willing and able to sign the marriage license.

Beyond those qualifications, what other business does any level of government have in our union?  If the state is supposed to be blind to color, gender and/or age in every other aspect of our lives, why does it matter whether both parties have innies or outies?

As long as both parties understand that they will become legally obligated to each other and to any children brought into the union, why does the gender of each part of the couple matter?  For that matter, why just a couple?  If all parties to the union are legally competent to enter into the contract, why does it matter to the state how many people unite?

I can't for the life of me understand how a committed same sex couple marrying threatens the idea of marriage any more than notable traditional marriages like those involving people like Larry King, the late Elizabeth Taylor, Britney Spears or whichever Kardashian it was that separated from her spouse before the ink on the license had dried.

Maybe I don't see it because I didn't have a traditional wedding.