David Rivkin and Lee Casey, writing in the Wall Street Journal, are Washington D.C.-based attorneys who served in the Department of Justice during the Ronald Reagan and George H.W. Bush administrations:
Marriage is unlike any other governmental benefit. License to marry carries with it far more than mere permission, as in obtaining a license to drive or practice a profession. The reason that gay-rights supporters are so determined to achieve equal status for same-sex unions, and the reason that so many others vigorously oppose that recognition, is that marriage is an affirmative statement of societal approval.
Congress took account of this fact in enacting DOMA, and of the fact that large majorities of Americans still oppose recognition of same-sex marriages. Significantly, most Americans do not oppose some other form of legal recognition for same-sex couples that isn't called marriage.
DOMA recognizes and protects the unique constitutional role of the states in deciding these issues. It is through the democratic process within the states that a genuine and lasting resolution to the question of same-sex marriage can and should be found. Today, five states and the District of Columbia permit same-sex marriage. Another four states recognize gay marriages performed in other jurisdictions, and 41 states do not recognize such unions. DOMA protects both legal regimes. (source)