Personally I think it's probably a good move by Obama to debate him on gay marriage. I think a majority of the country at this point is probably sympathetic to gay marriage & recognizes having it be illegal is an archaic and dying stance. I don't think Obama's going to break his back trying to use his influence to help legalize gay marriage, rather leave that up to the states, but at least he won't be the guy up there repeating the phrase "I believe marriage should be between a man and a woman", over and over. People do care about social issues as well.
I understand your point though, both candidates are fundamentally pretty bad in terms of civil liberties, with the most frightening example of which coming in the form of joint support of the infamous NDAA. I think any libertarian would blow both of them out of the water in this regard, & I'd give my left nut to be able to see Gary Johnson participate in the civil liberties debate. At least to me Gary would blow them out of the water, I'm very curious as to how the whole nation would react though. Libertarians are generally in favor of legalizing Drugs, Gambling, & Prostitution (which I am as well), but I don't know if the majority of America is ready for that kind of freedom just yet; I think most Americans are still inoculated with the notion that a Drug-War is in our interests. Nonetheless It's an idea I'd at least like to see permeate the lexicon of American thought.