Some of the nation's hottest social issues have played out in the esoteric, legal theory dominated domain of "choice of law." Usually, a court simply applies its own domestic laws to the facts before it, but not always.
One of the oldest of these disputes continues to play out. About half of the states permit first cousins to marry. About half do not. Louisiana, bad jokes about the South notwithstanding, do not permit first cousins to enter into marriages. But, what if first cousins lawfully marry in another state that does recognize first cousin marriages? Will Louisiana law recognize the marriage? The answer is yes.
They aren't constitutionally bound to do so. While recognition of foreign marriage certificates as valid if valid where the marriage was entered into is universal (and required by the full faith and credit clause for marriages entered into within another state), the marriage itself can be denied recognition if it violates a strong public policy of the state.
Recognition of common law marriages validly entered into in other states has also been nearly universal, although there is dispute over whether this flows from the full faith and credit clause of the constitution, or from comity (i.e. voluntary respect for foreign practice). Colorado is currently one of a handful of states whose laws recognize foreign common law marriages only if they would not be statutorily barred in Colorado on account of minority. A 17 year old's common law marriage in Kansas, isn't valid in Colorado. Colorado's law was enacted to prevent common law marriage to authorize what would otherwise be considered child sexual abuse involving girls to young to meaningfully consent to marriage.
Historically, bigamous and polygamous marriages, and sibling marriages validly entered into elsewhere have not been recognized under the public policy exception. Interracial marriages prior to the U.S. Supreme Court ruling in Loving v. Virginia, and cousin marriages have produced mixed rulings, some validating them despite being illegal locally, and others validating them.
Fights over the validity of foreign divorces, in places like Nevada and Mexico, have likewise spawned considerable choice of law litigation. Interstate child custody jurisdictional fights were a choice of law pit, until uniform laws and a federal law drew the line much more clearly, although internationally child custody fights remain deeply problematic.
The big open question now is gay marriage. A few states allow actual gay marriages. Many more states, Colorado among them, permit domestic partnerships with rights and responsibilities identical to marriage for purposes of state law.
Federal law currently purports to remove gay marriage from the scope of the obligations of states to honor under the full faith and credit clause of the constitution, and to prohibit recognition of gay marriage for federal law purposes like federal income taxation and social security benefits.
But, federal law isn't entirely a negative factor. Recent changes to federal tax laws improve the situation of domestic partners seeking to inherit retirement assets. The usually dreaded specter of ERISA pre-emption has prevented anti-gay provisions of state law from being applied to ERISA covered employee benefits that cover gay couples as a matter of employer good will, and their ability to grant domestic partnership benefits has provided a venue for municipal governments and pro-gay rights groups to pressure employers to grant such benefits.
There are already a trickle of cases governing recognition of foreign gay marriages and foreign domestic partnerships, and there are likely to be many more in almost every state as time passes. These cases will look to cases like the Louisiana case on cousin marriage, as they craft rules for gay marriages. The cases are likely to be all over the map, because the fundamental test of the existence of a "strong public policy" on the issue, is one that inherently involves some degree of judicial legislation.