Okay so I did a quick search on Obama +"Sex Tape", and there are apparently some YouTube videos of a guy who says he did cocaine with Obama and performed oral sex on him in 1999...
Obama Drugs and Gay Sex in Limo video
The guy seems like a crackpot, but you would think Obama or the Democratic Party would take some action since the one video which I saw (and there are others), already has hundreds of thousands of views, and other people have independently interviewed and broadcast this guy's claims (search his name +Obama and there are thousands of links already).
This is a low blow, no matter which way you look at it...
ADG