Eliminate marriage as a state recognized institution, to be replaced by an identical institution called Civil Unions or Domestic Partnerships. This new institution should have all the same benefits granted by the current institution of marriage, but would be available to all people, regardless of sexuality or gender of their partner.
An issue that America has been dealing with for some time now, is whether same-gender Americas should have the right to marry under law. The problem is that by having marriage as a state institution, you are not separating church and state. Making all couples, regardless of their genders, enter civil unions would allow religions to performs their own rights of union or marriage, while separating those unions from the state given rights to domestic unions.