Some fitness fads require sporty gear and equipment, but the practice of yoga requires only the bare essentials: loose clothes, a mat and time to do the exercises. The latest trend in yoga requires even less. We're not talking about aqua yoga, done in a pool, or disco yoga, set to dance tunes, or "boga," boxing yoga, done with gloves. No, a San Francisco community center is offering naked yoga, where bare essentials means just that: Men and women are completely nude during the 90- minute class. This is not the invention of "naked yoga guy" George Monty Davis, who...