I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.I am pretty sure that oath does not mean "I will support and uphold whatever laws I like and approve of". Failure to uphold the law is an impeachable offense. It is not the role of the Executive branch to create law, but to enforce law. Whatever you may think of the Defense of Marriage Act, it is the law of the land until it is repealed by Congress or repudiated by the Courts.
It is not the roll of the Justice Department or the President to pick and choose what laws to enforce.
Example #9,654 of why Democrats do not understand the Constitution.