The new question relating to same sex marriage is not "who decides," but who is married. The federal government and many states have historically relied on the place-of-celebration rule, or lex loci celebrationis, to determine who is married. This ensures that married couples do not lose their marital status simply because they travel across state borders or relocate to a new home. Under lex loci celebrationis, if the marriage is legally valid where it was celebrated, then the marriage is legally valid everywhere else.
This article address the most pressing unresolved question of United States v. Windsor: Will the federal government use the morphing and varied state definitions of marriage to asses which couples are legally married, or will it use a singular federal criterion, such as lex loci celebrationis? Resort to any paradigm excepting "the place of celebration" carries serious constitutional consequences. If same-sex marriages are evaluated under guidelines distinct from opposite-sex marriages, an equal protection challenge looms large. If federal recognition of same-sex marriages depends on remaining in a state that sanctions same-sex unions, the constitutional right to travel is impaired. These looming constitutional questions can easily be resolved by relying on lex loci celebrationis to determine who, for federal purposes, is married.
Something to [Lex Loci] Celebrationis: Federal Marriage Benefits following United States v. Windsor,
41 Hastings Const. L.Q. 41
Available at: https://repository.uchastings.edu/hastings_constitutional_law_quaterly/vol41/iss1/2