It shouldn't be, and indeed, nowhere in the Constitution does it allow anyone power to delegate on 'moral' issues. But the question is what is a moral issue and what is not. I personally do not believe that gay marriage should be an issue at all. They are citizens of the United States, they are allowed the RIGHT to marry, because MARRIAGE is not a PRIVILEGE.
The President, the spearhead of the Nation, is a representation of Us, and therefore Our morals. I don't think the President is responsible of solving moral dilemmas, and to think that s/he can is ridiculous, because they will undoubtly exist and even multiply as our advancements in science and our encroachment into uncharted territories continues.
It is not the 'job' of the President to solve moral issues, but in the end it does end up being so. That's why is is so important who we intrust such power to. That's why, so many times, religion and beliefs do become an issue when deciding on who represents us. Morals are fundamental in determining law, that's why so many are concerned with electing a President with 'their' morals, which can inturn be construed as 'their' religious belief.