To suggest that a state is the locus of new definitions of rights is a very dangerous precedent: we have seen in the past how state definitions of rights can change radically (examples here and here), and so the history of philosophical reflection suggests that rights must be located not in the legislative or judicial decisions of states, but rather in the thoughtful consideration of reality accessible through the application of reason over history. Aristotle, the Stoics, Christian scholastics, Muslim philosophers, and Enlightenment political theorists all held to this view of what came to be known as Natural Law. It is an imperfect doctrine because of the imperfection of human reason, but it is far superior to the vagaries of intra-state politics, because it locates rights not in a group vote but in a careful discernment of an unchanging reality about the human person. And because history teaches us that biases can pervade even for a generation or two, it is important to carefully consider the judgments of generations now far removed from our own. The claim to a "right" to gay marriage is suspect precisely because this right has no history.
Second, marriage. States have an interest in promoting all sorts of love relationships: between parents and children, among extended families, between friends, and so on, for love relationships promote social harmony and can be of themselves good, true, and beautiful. Yet in the main, only marriage between husbands and wives has, over history and around the world, been accorded legal recognition, because only those marriages involve overcoming the alienation between the two halves of the human family, and the procreation and rearing of children. States have an interest in reconciling the sexes, keeping fathers involved in families, and helping mothers through pregnancy, childbirth, and nurturing. The state's promotion of marriage invites adults to roles of particular social responsibility.
Positing a right to a new state-invented declaration of marriage will have negative consequences for a society. It takes a culture to support children, but it also takes a culture to support the hard work of communication to overcome the differences between men and women. Marriages between men and women are complex in ways that same-sex relationships are not. Expanding and changing what marriage means will likely have the consequence not of equalizing access to marriage, but helping to diminish the sense that it's worth bothering about.