Cross-branching

The interesting thing about the Obama administration's defense of the Defense of Marriage Act (DOMA) is that no matter how distasteful a law is, the Executive branch of the government is sworn to uphold it.
It is quite a big step for the Department of Justice to declare its opinion that this law is unconstitutional. Even when we know it is.

Also, please note the President has only retracted his willingness to defend this law in the courts, he explicitly states that his opinion will not affect its enforcement. (From the DOJ letter:

"...Section 3 will continue to be enforced by the Executive Branch. To that end, the President has instructed Executive agencies to continue to comply with Section 3 of DOMA, consistent with the Executive's obligation to take care that the laws be faithfully executed, unless and until Congress repeals Section 3 or the judicial branch renders a definitive verdict against the law's constitutionality. This course of action respects the actions of the prior Congress that enacted DOMA, and it recognizes the judiciary as the final arbiter of the constitutional claims raised.")