SSM Advocates often claim same-sex couples have a right to marry because of "equal protection." Legal scholar William Duncan explains why that is not the case over at the highly-read SCOTUSblog:

"...The decisions the Court will be reviewing have embraced something like a “substantive equal protection” doctrine.

... The Supreme Court now has an opportunity to wave the lower courts off this program of social engineering through a results-oriented jurisprudence.

The Court has appropriately cabined the more expansive applications of substantive due process so that courts may not create “rights” unknown to text, history and tradition. The concerns prompting this curb are also present in an attempt to impose a result with no mooring in Constitutional provisions, practice, or precedent through the Equal Protection Clause. Such a program ends an ongoing debate with complex and sensitive implications for family policy, religious freedom, federalism, etc.

... The justification for a novel application of equal protection has been that normal procedures for lawmaking are hopelessly inadequate for protecting certain identifiable groups. This is manifestly not the case in the marriage context. There is no reason, far from it, to believe the people of the states cannot, directly or through their representatives, appropriately respond to claims that marriage ought to be redefined. The claim that advocates of redefining marriage are politically powerless does not ring true."