From the decision:

The State of Utah has provided no evidence that opposite-sex marriage will be affected inany way by same-sex marriage. In the absence of such evidence, the State’s unsupported fears and speculations are insufficient to justify the State’s refusal to dignify the family relationships of its gay and lesbian citizens. Moreover, the Constitution protects the Plaintiffs’ fundamental rights, which include the right to marry and the right to have that marriage recognized by their government. These rights would be meaningless if the Constitution did not also prevent the government from interfering with the intensely personal choices an individual makes when that person decides to make a solemn commitment to another human being. The Constitution therefore protects the choice of one’s partner for all citizens, regardless of their sexual identity.