Let us look at the world from the same sex couples perspective. There is not much to say. They have a right to be who they are in their private lives and deserve the same consideration as any other tax payer gets. That is it. It is nobody's business telling anybody what to do. My home is my castle.
But then that is not it. Really. Humm... here it goes.
Nature intended sex and sexual act for the purposes of procreation. That is true. But nature does not work in precise ways. It is probabilistic and often accepts a lot of variability on almost all its manifestations. In general that variability produces diversity and beauty in the world. There is nothing evil about it and we do not have to be paranoid about loosing our identity of a generally heterosexual world. The world is not going to cease to be largely hetrosexual. Nature will not allow for that to happen. While it can be granted that deviation from the common is obviously uncommon and will never become the norm, yet people are justifiably homophobic for one good reason, even though we know that existence of diversity is very normal in nature and is not a cause for any alarm.
Destruction of the social fabric as some claim same sex marriage will bring is purely imaginary because the fabric is not static. It is an ever changing one as we continue to manipulate nature and create new and newer equilibrium(s) from one society to another. We live in an inherently socially constructed reality unlike the animal kingdom. And none is better than the other except where we reduce the sum total of all "real" physical pain in the human world, which I am not sure we have done, anyways.
Are children of same sex couples going to be deviants? Are they going to experience a poor quality existence ? I can't think of that as a more important criterion than one that requires stable healthy and loving relationships in the family. I would put my attention on solving that problem first. All else being equal, will an average child be better off in an average environment rather than an uncommon one? What is the harm in saying yes to that and setting aside rules for adoption that way. Mother gets a preferential treatment in divorce proceedings in general then why not homosexual couples.
What is the fear then? Is our child going to become homosexual by association? That fear can be terrifying to some because it is antithetical to the very purpose of human existence - procreation. Some of us more prone to visceral reactions do react that way. Homophobia is real for that reason and that reason alone. However, the truth is that there is plenty of explaining that parents need to do to their children. One more will not kill you. If anything, it will only make a better case for understanding how nature works.
I am not sure what the supreme court justices are thinking (or not). But I think we are simply dragging the inevitable and inexorable milestones of the human journey.