A declaration for or against gay marriage would have symbolic -- not legal -- value.
President Obama can order the assassination of an American citizen but he cannot order the performance of a same-sex marriage. The problem isn't his lack of power in the second case but his appropriation of power in the first. Yet his equivocation about marriage may matter in November at least a little; stating a position for or against it could matter a lot; his assassination authority will likely matter not at all.
Make war, not love. The targeted assassination of citizens merely suspected of terrorism enjoys popular support (a 79 percent approval rating), while same sex marriage passionately divides us, generating heated controversies that the president hesitates to touch. It's an ugly portrait of post-9/11 America: More people are concerned with restricting their neighbor's right to marry than the president's power to kill.
It's no wonder his position on marriage is "evolving;" so is the position of the public. Besides, if Obama declared for or against gay marriage, his declaration would have symbolic, not legal value. His administration has already exercised the primary legal power it possesses in this debate by declining to defend the Defense of Marriage Act, which denies federal recognition and federal benefits to same sex couples married under state law. DOMA is now subject to a strong 14th Amendment challenge involving a defense of states' rights, not an invocation of federal, much less presidential, power.