DOMA shouldn't be defended, it is bad law that creates work for lawyers but is a mess for citizens. It is bad to allow same-sex marriage in some states, and useless not to allow it in others. There are aspects of marriage that are fundamental rights which are due to every citizen of every state, and fundamental rights which states should be prohibited from abridging. Even Section 3 of DOMA is bad law, it fails to properly protect or define marriage, and only allows the government to save money on spousal benefits.
DOMA should not be defended, it should be replaced with the three laws of the Egg and Sperm Civil Union Compromise which would truly defend marriage:
1) Prohibit the conception of children by any means other than the union of unmodified gametes.
2) Protect the inherent right of marriage to conceive offspring.
3) Federally recognize state Civil Unions that are defined as "marriage minus conception rights"
The first two laws are urgently necessary and would have immediate and long term benefits, and would avoid the costs and ethical issues of allowing use of modified gametes and regulating conception rights separately from marriage. It is really hard to justify not enacting those first two laws, or delay enacting them one minute.
The third law is not as necessary as far as society is concerned, but it still would have immediate and long term benefits for society, and would certainly benefit the thousands of same sex couples that lack recognition right now, and I think would be fair and compassionate and not cost too much.
DOMA could be replaced with the Egg and Sperm Civil Union Compromise with minimal deliberation and would achieve Obama's and the vast majority's goals in a principled and permanent and politically acceptable way:
1) It would preserve marriage as a man and a woman in every state.
2) It would allow Civil Unions in every state with fully equal protections and rights, except of course for the right to procreate genetically-related offspring together.
3) It would prohibit cloning, human-animal children, and producing children for same-sex couples using lab created artificial sperm or egg.
Who in Congress is going to say that any of those laws would be bad? That the status quo is better? I know that the libertarians here and at GOProud are going to object, but they'll look silly doing so.