It wasn't just a debate though. Getting beat is one thing - as a sports fan you know there are games where your team is just outmatched, gives their all but still loses. But there are also games where your team completely gives up (like say, Kobe against the Suns a few years ago in the playoffs). No one respects that. I think Obama went in with an attitude that because he believes debates are meaningless, he wasn't going to take it seriously. Voters saw someone who looked at times disengaged, at times outmatched, and at times feckless. In short: unpresidential.
Before the debate most voters may have liked or disliked Obama, but believed he was doing his best and working hard. After that debate there have been multiple people questioning that. If he can't fight to save his own job, why should voters vote for him? Likewise from a dem perspective, if he can't fight for himself why should they fight for him? Hence why you had some voters turned off by it, and other voters completely demoralized by it.