Marriage is a private and personal relationship between two (or more?) people. Government should have no involvement in sanctioning, granting special rights to, or banning any marriage between consenting adults. Those involved in marriage, whether they be heterosexual or homosexual, are the only ones that have any valid say in what they constitute to be their marriage.
While marriage has been around for most of human civilization, governments within the United States only began their involvement within the last 100 years. Originally, the purpose of licensing marriages was to prevent interracial relationships. Since this time, governments have continued dictating under what conditions and to whom two people may marry. For as long as the government is in the business of handing out marriage licenses, the issuance of licenses should be at the discretion of the recipients, not the politicians or bureaucrats.
The ultimate solution is to completely remove government involvement from marriage, and allow adults to make their own decisions about their relationship arrangements.