He wasn't naked: this is what John Cena was actually wearing under the envelope at the Oscars. The former wrestler was wearing a kind of flesh-colored mini swimsuit that covered both the front and the back, but left his entire body exposed on the side.
The solution he found was to copy a trick from celebrities like Kim Kardashian, when they cover their breasts with adhesive cloth tape to control impossible necklines. In this way, it didn't matter if he moved more than necessary, stumbled or dropped the envelope: everything was secured and stayed in his place.