I strongly doubt there is any merit in the claim that marriage was principally designed for procreation. In early societies marriage was seen as a way of providing a companion and a work partner in securing a subsistence. So it was mainly a means of providing comfort and protection. Having children and building a family was just a natural consequence of that. Even in the American frontier men got married in order to have a companion and someone to cater to their domestic needs. It was widely known that the frontier prostitutes would take money to sleep with you but you couldn't pay them enough to come home with you and wash your undies. In the Bible Eve was created as a companion for Adam, so that he wouldn't be alone. And in the New Testament Apostle Paul endorses marriage for those unable to live a life of chastity and solitude. (Christianity's position on homosexuality is another matter but many churches are having to rethink their positions on that too).
Clarence Thomas' claim that the state cannot deny a person dignity seems confused at best. Is it not a State's duty to protect the freedom of all citizens. And would a denial of access to a freely available social institution not reduce a person's standing in society? To be stripped of your justly entitled rights is an affront to personal dignity. Justice Thomas will do well not to confuse pride with dignity. Dignity comes from elevation. You cannot be truly elevated if you are relegated to a lower status than others around you.
As Michel de Montaigne wrote, "marriage is like a cage; one sees the birds outside desperate to get in, and those inside equally desperate to get out". However marriage turns out; the choice should be all people's to make.