Yes, the administration usually defends the existing law against constitutional challenges. Here is Gibbs's defense:
Today, the Department of Justice made a filing in a legal challenge to the Don’t Ask, Don’t tell (DADT) policy, as it traditionally does when acts of Congress are challenged. This filing in no way diminishes the President’s firm commitment to achieve a legislative repeal of DADT ... The President, along with his Administration, will continue to work with the Senate Leadership to achieve a legislative repeal of DADT as outlined in the NDAA this fall.
But does no one see the irony of the Log Cabin Republicans now suing Barack Obama in order to stop him firing people solely because they are gay? I wish I could feel some sympathy for the president on this issue. But he is making Bill Clinton look like a model of principle - and his rank cowardice on civil rights is one of the greatest disappointments of his first two years.