The so-called "Defense of Marriage Act" (PL 104-199), began life as H.R.3396, and was introduced by Bob Barr, R-GA, who turned out to have most likely cheated on his second wife with the woman who would become his third wife (yeah, he knows something about marriage all right).

It has three sections.

Section one is the title of the act, which is generally shortened to its acronym, DOMA (I can't call it by its full, official name).

Section two relieves states, territories and possessions of the U.S. (and "Indian tribes") from having to accept same-sex marriages performed in other states. It doesn't outright prohibit states from accepting them, merely lets them refuse to do so. This is in direct conflict with Article IV, Section 1 of the U.S. Constitution, which begins: "Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State."

In other words, section two of DOMA seeks to let states not give "Full Faith and Credit" to same-sex marriages performed in other states; it is clearly unconstitutional.

Possibly in an attempt to justify this section, Section three "defines" marriage by saying, "In determining the meaning of any Act of Congress, or of any ruling, regulation, or interpretation of the various administrative bureaus and agencies of the United States, the word 'marriage' means only a legal union between one man and one woman as husband and wife, and the word 'spouse' refers only to a person of the opposite sex who is a husband or a wife." This is much more difficult to challenge constitutionally.

The tenth amendment states, "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." One could argue that since defining, creating, and enforcing civil marriages, like other civil laws, is a power "not delegated to the United States by the Constitution, nor prohibited by it to the States," the federal government has no constitutional right to define civil marriage.

As I see it, the potential problem with this argument is that DOMA only seeks to define marriage insofar as it applies to Act[s] of Congress and dealings with federal agencies, not to states, so it allows states to define marriage as they see fit.

The other argument against the constitutionality of this section of DOMA is the ninth amendment, which states, "The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people." One could argue that the right to enter into a civil contract known as marriage cannot be denied or disparaged by the federal government. Indeed, the U.S. Supreme Court, since the days of Chief Justice Renquist, has held a much stronger view of this amendment, applying it to strike down a lot of federal laws and actions; if the Roberts court follows this pattern, it could conceivably be an argument.

The problem seems to me to be that neither argument against Section 3 of DOMA is necessarily certain, which is why congressional action to repeal it would be a much better solution than trying to have the courts strike it down.