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.
States have always enjoyed exclusive power to marry people and Congress has always recognized marriages legalized by the states, until it passed DOMA in 1996. The most promising and, so far, successful challenge to DOMA, Gill v. OPM, (initiated by GLAD and recently argued before the 1st Circuit) relies partly on traditional Congressional deference to state marriage laws. (The Supreme Court, not Congress, invalidated state bans on inter-racial marriages.)
For over 200 years, GLAD explains, Congress expressed no interest in rejecting any state definition of marriage and so lacks a rational basis for its recent interest in rejecting one definition of marriage and discriminating against gay couples legally wed by their states. Massachusetts Attorney General Martha Coakley argues additionally that DOMA is an unconstitutional violation of state sovereignty over marriage.
If the challenge to DOMA is successful, states will regain their power to decide whether or not to confer equal marriage rights on gay people. The president will be legally irrelevant to this process. If the challenge to DOMA fails, its repeal will be left to Congress, which seems unlikely to act in the near future and even less likely to act under the influence of Barack Obama, if he wins a second term. The Supreme Court could eventually determine the constitutionality of gay marriage bans, if it hears an appeal in Perry v. Brown. But the Court is only indirectly influenced by the president, whose judicial nominees are sharply limited by Congress.
So, as an advocate of equal marriage, I really don't care what the Obama thinks about it. I worry about the powers he appropriates, not the opinions he abdicates.