There is none and in fact there cannot be.
Marriage, as a legal entity, exists because the state created it, and exists so long as the laws that create it are in place - repeal those laws, and marriage as a legal institution, ceases to ecist.
As such, marriage can only be a privilege as the state cannot create rights and rights cannot be repealed.
Allowing some to marry, and others not to, and then giving preferential treatment to those people who are married, is clearly in violation of the Equal Protection Clause of the 14th Amendment.
Whether marriage is a "right" or not, matters not. Giving benefits to one group of people and not allowing another group of people to enjoy said benefits is unconstitutional, and therefore illegal.
Therefore:
In any state where homosexual marriage is illegal, the law is clear:
All Federal rights, protections, and benefits normally applied to married couples must be nullified for ALL married couples, married in that state.
This is a no brainer. The Federal part can pretty much be enacted by the Executive Branch effective immediately.
This will of course include all tax breaks for families and married couples in these states.