So your position is anyone can marry anyone, regardless of whether society approves, or thinks it is legitimate? What's to stop people from having group marriages of 3 people? Or 30? Or to stop fathers from marrying their daughers? (Or sons!)
I know, I know, no one likes the slippery slope argument. My point is just this: Can society really say that it can, and should place limits on polygamy or incest, or even a minimum marriage age, and then say that it cannot place other limits on other groups? (Like gays) I'm not equating gays with polygamists, or people who commit incest, or anything like that. I'm just saying that most people would support banning polygamy, or incest, or pedophilia. Why is it legitimate to ban certain forms of "marriage" but not others? A little consistency would be nice.