We can surely say changes have been made in America, but they have ALWAYS started at the state level then eventually influenced the president. Civil rights, womens lib, gay marrigae, legalized medicinal marijuana, gun control, have all been state issues before any predient has taking it to task to try and fix things.
Even admitting your premise, it's not the case that the choice of President is immaterial. Because how he decides to fix these things raised at state level depends on who s/he is or what he thinks.
Gay marriage was first raised at state level? Sure. Now think about how. Some states have made it legal, some states haven't, and some states have changed their constitutions to say that it should never be legal. And then some politicians on the national stage have come out in favor of it, and some against it. Obama is in favor of it, Romney is against it, and some of the Republicans in the primaries would have supported a constitutional amendment taking that power away from the states.
It's not unimportant what the President thinks. He has the power of veto. If someone want to (literally) make a federal case out of some issue, then the President can stop them from doing it, if they don't have the votes to over-ride.