Marriage isn't just a social contract between 2 people, it's a contract between you society and that couple. That's why. I can make any arrangement I want with any number of women, but when you get married there's a whole lot of cultural and legal responsibility on the part of the state and teh couple that come along with that.
R Kelly can sit in his mansion with 30 on-demand hos willing to do whatever he says, but the state's not recognizing any of them as his wife.
THis is my problem with gay marriage, it asks society to treat a gay union the same as a biological coupling where in the latter case there are thousands of years of tradition and reasoning behind the state recognizing their union.