Songs by Madonna and Prince inspired Cliff Boyce to swing his hips and rub his thighs. But it was Christina Aguilera’s ‘Dirrty’ that got the aerobics instructor stripping down to his red bikini briefs. The dramatic dropping of his stretchy jeans and shiny vest barely raised an eyebrow. In fact, this booty-shaking class at the Buckhead Crunch Fitness club was wondering what took him so long. It was, after all, 20 minutes into “cardio striptease,” the latest aerobics-workout fad to flirt its way into Atlanta.