Constitutional Puzzlement

Presidential administrations do a great many things that mystify me, but reading Eric Holder's statement on the President's recent decision with regard to DOMA, I am more mystified than usual. It is entirely reasonable for a President who thinks a law unconstitutional to refuse to defend it in court; that is the only way to fulfill the obligation of his oath. What I don't understand is how a President, once he has made this decision, can also continue to enforce the law, since an unconstitutional law, being void, shouldn't fall under the Constitutional requirement to execute the Laws faithfully -- that means executing the Constitution faithfully above all else, but if a law is unconstitutional, the Constitution can't be faithfully enforced as long as the unconstitutional law is also enforced. How does one have one's cake and eat it, too? What conceivable argument can be given for the claim that one can constitutionally enforce what one regards as unconstitutional?