Defending DOMA is simple. As ADF's Jordan Lorence points out, there's controlling Supreme Court precedent on the question of same-sex marriage:

"A Minnesota state trial judge in Minneapolis showed President Obama and the Department of Justice at least one major way to defend a law defining marriage as one man and one woman. Hennepin County District Court Judge Mary Dufresne on Monday dismissed a lawsuit by those seeking to redefine marriage to include same sex couples.

According to a media report, Judge Dufresne relied on the Minnesota Supreme Court’s 1971 decision in Baker v. Nelson, which the U.S. Supreme Court summarily affirmed in 1972. That case, brought by a same-sex couple seeking to marry in Minnesota, raised every major constitutional challenge to Minnesota’s marriage law. They lost in court and appealed to the U.S. Supreme Court, where they lost as well.

... The Minnesota case will be appealed to the state Court of Appeals and probably to the Minnesota Supreme Court.  I fully expect the lawsuit seeking to redefine marriage to lose.  This decision by Judge Dufresne provides a clear lesson to President Obama that there are a number of reasonable legal arguments that the Department of Justice can make to defend a marriage law like DOMA." (source)